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/language/src/buffer.rs

@@ -503,9 +503,9 @@ pub enum CharKind {
 
 impl Buffer {
     /// Create a new buffer with the given base text.
-    pub fn new<T: Into<String>>(replica_id: ReplicaId, id: BufferId, base_text: T) -> Self {
+    pub fn local<T: Into<String>>(base_text: T, cx: &mut ModelContext<Self>) -> Self {
         Self::build(
-            TextBuffer::new(replica_id, id, base_text.into()),
+            TextBuffer::new(0, cx.entity_id().as_non_zero_u64().into(), base_text.into()),
             None,
             None,
             Capability::ReadWrite,
@@ -517,10 +517,10 @@ impl Buffer {
         remote_id: BufferId,
         replica_id: ReplicaId,
         capability: Capability,
-        base_text: String,
+        base_text: impl Into<String>,
     ) -> Self {
         Self::build(
-            TextBuffer::new(replica_id, remote_id, base_text),
+            TextBuffer::new(replica_id, remote_id, base_text.into()),
             None,
             None,
             capability,