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/movement.rs

@@ -570,7 +570,6 @@ mod tests {
     use language::Capability;
     use project::Project;
     use settings::SettingsStore;
-    use text::BufferId;
     use util::post_inc;
 
     #[gpui::test]
@@ -870,13 +869,7 @@ mod tests {
 
             let font = font("Helvetica");
 
-            let buffer = cx.new_model(|cx| {
-                Buffer::new(
-                    0,
-                    BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                    "abc\ndefg\nhijkl\nmn",
-                )
-            });
+            let buffer = cx.new_model(|cx| Buffer::local("abc\ndefg\nhijkl\nmn", cx));
             let multibuffer = cx.new_model(|cx| {
                 let mut multibuffer = MultiBuffer::new(0, Capability::ReadWrite);
                 multibuffer.push_excerpts(