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/text/src/text.rs

@@ -71,12 +71,19 @@ impl Display for BufferId {
     }
 }
 
+impl From<NonZeroU64> for BufferId {
+    fn from(id: NonZeroU64) -> Self {
+        BufferId(id)
+    }
+}
+
 impl BufferId {
     /// Returns Err if `id` is outside of BufferId domain.
     pub fn new(id: u64) -> anyhow::Result<Self> {
         let id = NonZeroU64::new(id).context("Buffer id cannot be 0.")?;
         Ok(Self(id))
     }
+
     /// Increments this buffer id, returning the old value.
     /// So that's a post-increment operator in disguise.
     pub fn next(&mut self) -> Self {