Add prompt library (#11910)

This PR adds a Prompt Library to Zed, powering custom prompts and any
default prompts we want to package with the assistant.

These are useful for:

- Creating a "default prompt" - a super prompt that includes a
collection of things you want the assistant to know in every
conversation.
- Adding single prompts to your current context to help guide the
assistant's responses.
- (In the future) dynamically adding certain prompts to the assistant
based on the current context, such as the presence of Rust code or a
specific async runtime you want to work with.

These will also be useful for populating the assistant actions typeahead
we plan to build in the near future.

## Prompt Library

The prompt library is a registry of prompts. Initially by default when
opening the assistant, the prompt manager will load any custom prompts
present in your `~/.config/zed/prompts` directory.

Checked prompts are included in your "default prompt", which can be
inserted into the assitant by running `assistant: insert default prompt`
or clicking the `Insert Default Prompt` button in the assistant panel's
more menu.

When the app starts, no prompts are set to default. You can add prompts
to the default by checking them in the Prompt Library.

I plan to improve this UX in the future, allowing your default prompts
to be remembered, and allowing creating, editing and exporting prompts
from the Library.

### Creating a custom prompt

Prompts have a simple format:

```json
{
  // ~/.config/zed/prompts/no-comments.json
  "title": "No comments in code",
  "version": "1.0",
  "author": "Nate Butler <iamnbutler@gmail.com>",
  "languages": ["*"],
  "prompt": "Do not add inline or doc comments to any returned code. Avoid removing existing comments unless they are no longer accurate due to changes in the code."
}
```

Ensure you properly escape your prompt string when creating a new prompt
file.

Example:

```json
{
  // ...
  "prompt": "This project using the gpui crate as it's UI framework for building UI in Rust. When working in Rust files with gpui components, import it's dependencies using `use gpui::{*, prelude::*}`.\n\nWhen a struct has a `#[derive(IntoElement)]` attribute, it is a UI component that must implement `RenderOnce`. Example:\n\n```rust\n#[derive(IntoElement)]\nstruct MyComponent {\n    id: ElementId,\n}\n\nimpl MyComponent {\n    pub fn new(id: impl Into<ElementId>) -> Self {\n        Self { id.into() }\n    }\n}\n\nimpl RenderOnce for MyComponent {\n    fn render(self, cx: &mut WindowContext) -> impl IntoElement {\n        div().id(self.id.clone()).child(text(\"Hello, world!\"))\n    }\n}\n```"
}
```


Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

docs/src/assistant-panel.md

@@ -133,3 +133,47 @@ You can use Ollama with the Zed assistant by making Ollama appear as an OpenAPI
   ollama
   ```
 5. Restart Zed
+
+## Prompt Manager
+
+Zed has a prompt manager for enabling and disabling custom prompts.
+
+These are useful for:
+
+- Creating a "default prompt" - a super prompt that includes a collection of things you want the assistant to know in every conversation.
+- Adding single prompts to your current context to help guide the assistant's responses.
+- (In the future) dynamically adding certain prompts to the assistant based on the current context, such as the presence of Rust code or a specific async runtime you want to work with.
+
+You can access the prompt manager by selecting `Prompt Library...` from the assistant panel's more menu.
+
+By default when opening the assistant, the prompt manager will load any custom prompts present in your `~/.config/zed/prompts` directory.
+
+Checked prompts are included in your "default prompt", which can be inserted into the assistant by running `assistant: insert default prompt` or clicking the `Insert Default Prompt` button in the assistant panel's more menu.
+
+### Creating a custom prompt
+
+Prompts have a simple format:
+
+```json
+{
+  // ~/.config/zed/prompts/no-comments.json
+  "title": "No comments in code",
+  "version": "1.0",
+  "author": "Nate Butler <iamnbutler@gmail.com>",
+  "languages": ["*"],
+  "prompt": "Do not add inline or doc comments to any returned code. Avoid removing existing comments unless they are no longer accurate due to changes in the code."
+}
+```
+
+Ensure you properly escape your prompt string when creating a new prompt file.
+
+Example:
+
+```json
+{
+  // ...
+  "prompt": "This project using the gpui crate as it's UI framework for building UI in Rust. When working in Rust files with gpui components, import it's dependencies using `use gpui::{*, prelude::*}`.\n\nWhen a struct has a `#[derive(IntoElement)]` attribute, it is a UI component that must implement `RenderOnce`. Example:\n\n```rust\n#[derive(IntoElement)]\nstruct MyComponent {\n    id: ElementId,\n}\n\nimpl MyComponent {\n    pub fn new(id: impl Into<ElementId>) -> Self {\n        Self { id.into() }\n    }\n}\n\nimpl RenderOnce for MyComponent {\n    fn render(self, cx: &mut WindowContext) -> impl IntoElement {\n        div().id(self.id.clone()).child(text(\"Hello, world!\"))\n    }\n}\n```"
+}
+```
+
+In the future we'll allow creating and editing prompts directly in the prompt manager, reducing the need to do this by hand.