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/gpui/src/window/element_cx.rs

@@ -264,6 +264,18 @@ impl<'a> Context for ElementContext<'a> {
         self.cx.new_model(build_model)
     }
 
+    fn reserve_model<T: 'static>(&mut self) -> Self::Result<crate::Reservation<T>> {
+        self.cx.reserve_model()
+    }
+
+    fn insert_model<T: 'static>(
+        &mut self,
+        reservation: crate::Reservation<T>,
+        build_model: impl FnOnce(&mut crate::ModelContext<'_, T>) -> T,
+    ) -> Self::Result<crate::Model<T>> {
+        self.cx.insert_model(reservation, build_model)
+    }
+
     fn update_model<T, R>(
         &mut self,
         handle: &crate::Model<T>,