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/editor/src/editor.rs

@@ -1328,37 +1328,19 @@ impl InlayHintRefreshReason {
 
 impl Editor {
     pub fn single_line(cx: &mut ViewContext<Self>) -> Self {
-        let buffer = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                String::new(),
-            )
-        });
+        let buffer = cx.new_model(|cx| Buffer::local("", cx));
         let buffer = cx.new_model(|cx| MultiBuffer::singleton(buffer, cx));
         Self::new(EditorMode::SingleLine, buffer, None, cx)
     }
 
     pub fn multi_line(cx: &mut ViewContext<Self>) -> Self {
-        let buffer = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                String::new(),
-            )
-        });
+        let buffer = cx.new_model(|cx| Buffer::local("", cx));
         let buffer = cx.new_model(|cx| MultiBuffer::singleton(buffer, cx));
         Self::new(EditorMode::Full, buffer, None, cx)
     }
 
     pub fn auto_height(max_lines: usize, cx: &mut ViewContext<Self>) -> Self {
-        let buffer = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                String::new(),
-            )
-        });
+        let buffer = cx.new_model(|cx| Buffer::local("", cx));
         let buffer = cx.new_model(|cx| MultiBuffer::singleton(buffer, cx));
         Self::new(EditorMode::AutoHeight { max_lines }, buffer, None, cx)
     }