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/copilot/src/copilot.rs

@@ -1042,9 +1042,7 @@ mod tests {
     async fn test_buffer_management(cx: &mut TestAppContext) {
         let (copilot, mut lsp) = Copilot::fake(cx);
 
-        let buffer_1 = cx.new_model(|cx| {
-            Buffer::new(0, BufferId::new(cx.entity_id().as_u64()).unwrap(), "Hello")
-        });
+        let buffer_1 = cx.new_model(|cx| Buffer::local("Hello", cx));
         let buffer_1_uri: lsp::Url = format!("buffer://{}", buffer_1.entity_id().as_u64())
             .parse()
             .unwrap();
@@ -1062,13 +1060,7 @@ mod tests {
             }
         );
 
-        let buffer_2 = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                "Goodbye",
-            )
-        });
+        let buffer_2 = cx.new_model(|cx| Buffer::local("Goodbye", cx));
         let buffer_2_uri: lsp::Url = format!("buffer://{}", buffer_2.entity_id().as_u64())
             .parse()
             .unwrap();