Start on a database-backed prompt library (#12468)

Using the file system as a database seems like it's easy, but it's
actually a real pain. I'd like to use LMDB to store the prompts locally
so we have more control. We can always add an export option, but I want
the source of truth to be somewhere other than the file system.

So far, I have a PromptStore which is global to the application and can
be initialized on startup. Then there's a `PromptLibrary` which is
intended to be the root of a new kind of Zed window. I haven't actually
seen pixels yet, but I've sketched out the basics needed to create a new
prompt, save, etc.

Still lots to figure out but the foundations of being backed by a DB and
rendering in an independent window are in place.

/cc @iamnbutler @as-cii 

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant/src/prompts.rs

@@ -1,7 +1,95 @@
-mod prompt;
-mod prompt_library;
-mod prompt_manager;
+use language::BufferSnapshot;
+use std::{fmt::Write, ops::Range};
 
-pub use prompt::*;
-pub use prompt_library::*;
-pub use prompt_manager::*;
+pub fn generate_content_prompt(
+    user_prompt: String,
+    language_name: Option<&str>,
+    buffer: BufferSnapshot,
+    range: Range<usize>,
+    project_name: Option<String>,
+) -> anyhow::Result<String> {
+    let mut prompt = String::new();
+
+    let content_type = match language_name {
+        None | Some("Markdown" | "Plain Text") => {
+            writeln!(prompt, "You are an expert engineer.")?;
+            "Text"
+        }
+        Some(language_name) => {
+            writeln!(prompt, "You are an expert {language_name} engineer.")?;
+            writeln!(
+                prompt,
+                "Your answer MUST always and only be valid {}.",
+                language_name
+            )?;
+            "Code"
+        }
+    };
+
+    if let Some(project_name) = project_name {
+        writeln!(
+            prompt,
+            "You are currently working inside the '{project_name}' project in code editor Zed."
+        )?;
+    }
+
+    // Include file content.
+    for chunk in buffer.text_for_range(0..range.start) {
+        prompt.push_str(chunk);
+    }
+
+    if range.is_empty() {
+        prompt.push_str("<|START|>");
+    } else {
+        prompt.push_str("<|START|");
+    }
+
+    for chunk in buffer.text_for_range(range.clone()) {
+        prompt.push_str(chunk);
+    }
+
+    if !range.is_empty() {
+        prompt.push_str("|END|>");
+    }
+
+    for chunk in buffer.text_for_range(range.end..buffer.len()) {
+        prompt.push_str(chunk);
+    }
+
+    prompt.push('\n');
+
+    if range.is_empty() {
+        writeln!(
+            prompt,
+            "Assume the cursor is located where the `<|START|>` span is."
+        )
+        .unwrap();
+        writeln!(
+            prompt,
+            "{content_type} can't be replaced, so assume your answer will be inserted at the cursor.",
+        )
+        .unwrap();
+        writeln!(
+            prompt,
+            "Generate {content_type} based on the users prompt: {user_prompt}",
+        )
+        .unwrap();
+    } else {
+        writeln!(prompt, "Modify the user's selected {content_type} based upon the users prompt: '{user_prompt}'").unwrap();
+        writeln!(prompt, "You must reply with only the adjusted {content_type} (within the '<|START|' and '|END|>' spans) not the entire file.").unwrap();
+        writeln!(
+            prompt,
+            "Double check that you only return code and not the '<|START|' and '|END|'> spans"
+        )
+        .unwrap();
+    }
+
+    writeln!(prompt, "Never make remarks about the output.").unwrap();
+    writeln!(
+        prompt,
+        "Do not return anything else, except the generated {content_type}."
+    )
+    .unwrap();
+
+    Ok(prompt)
+}