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/copilot_ui/src/copilot_completion_provider.rs

@@ -272,7 +272,7 @@ mod tests {
     use indoc::indoc;
     use language::{
         language_settings::{AllLanguageSettings, AllLanguageSettingsContent},
-        BufferId, Point,
+        Point,
     };
     use project::Project;
     use serde_json::json;
@@ -729,20 +729,8 @@ mod tests {
 
         let (copilot, copilot_lsp) = Copilot::fake(cx);
 
-        let buffer_1 = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                "a = 1\nb = 2\n",
-            )
-        });
-        let buffer_2 = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                "c = 3\nd = 4\n",
-            )
-        });
+        let buffer_1 = cx.new_model(|cx| Buffer::local("a = 1\nb = 2\n", cx));
+        let buffer_2 = cx.new_model(|cx| Buffer::local("c = 3\nd = 4\n", cx));
         let multibuffer = cx.new_model(|cx| {
             let mut multibuffer = MultiBuffer::new(0, language::Capability::ReadWrite);
             multibuffer.push_excerpts(