Improve the ergonomics of creating local buffers (#10347)

This PR renames `language::Buffer::new` to `language::Buffer::local` and
simplifies its interface. Instead of taking a replica id (which should
always be 0 for the local case) and a `BufferId`, which was awkward and
verbose to construct, it simply takes text and a `cx`.

It uses the `cx` to derive a `BufferId` from the `EntityId` associated
with the `cx`, which should always be positive based on the following
analysis...

We convert the entity id to a u64 using this method on `EntityId`, which
is defined by macros in the `slotmap` crate:

```rust
    pub fn as_ffi(self) -> u64 {
        (u64::from(self.version.get()) << 32) | u64::from(self.idx)
    }
```

If you look at the type of `version` in `KeyData`, it is non-zero:

```rust
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct KeyData {
    idx: u32,
    version: NonZeroU32,
}
```

This commit also adds `Context::reserve_model` and
`Context::insert_model` to determine a model's entity ID before it is
created, which we need in order to assign a `BufferId` in the background
when loading a buffer asynchronously.

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

crates/assistant/src/assistant_panel.rs

@@ -31,7 +31,7 @@ use gpui::{
     StatefulInteractiveElement, Styled, Subscription, Task, TextStyle, UniformListScrollHandle,
     View, ViewContext, VisualContext, WeakModel, WeakView, WhiteSpace, WindowContext,
 };
-use language::{language_settings::SoftWrap, Buffer, BufferId, LanguageRegistry, ToOffset as _};
+use language::{language_settings::SoftWrap, Buffer, LanguageRegistry, ToOffset as _};
 use parking_lot::Mutex;
 use project::Project;
 use search::{buffer_search::DivRegistrar, BufferSearchBar};
@@ -1310,7 +1310,7 @@ impl Conversation {
     ) -> Self {
         let markdown = language_registry.language_for_name("Markdown");
         let buffer = cx.new_model(|cx| {
-            let mut buffer = Buffer::new(0, BufferId::new(cx.entity_id().as_u64()).unwrap(), "");
+            let mut buffer = Buffer::local("", cx);
             buffer.set_language_registry(language_registry);
             cx.spawn(|buffer, mut cx| async move {
                 let markdown = markdown.await?;
@@ -1398,11 +1398,7 @@ impl Conversation {
         let mut message_anchors = Vec::new();
         let mut next_message_id = MessageId(0);
         let buffer = cx.new_model(|cx| {
-            let mut buffer = Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                saved_conversation.text,
-            );
+            let mut buffer = Buffer::local(saved_conversation.text, cx);
             for message in saved_conversation.messages {
                 message_anchors.push(MessageAnchor {
                     id: message.id,