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/test/editor_test_context.rs

@@ -21,7 +21,6 @@ use std::{
         Arc,
     },
 };
-use text::BufferId;
 use ui::Context;
 use util::{
     assert_set_eq,
@@ -76,10 +75,9 @@ impl EditorTestContext {
     ) -> EditorTestContext {
         let mut multibuffer = MultiBuffer::new(0, language::Capability::ReadWrite);
         let buffer = cx.new_model(|cx| {
-            for (i, excerpt) in excerpts.into_iter().enumerate() {
+            for excerpt in excerpts.into_iter() {
                 let (text, ranges) = marked_text_ranges(excerpt, false);
-                let buffer =
-                    cx.new_model(|_| Buffer::new(0, BufferId::new(i as u64 + 1).unwrap(), text));
+                let buffer = cx.new_model(|cx| Buffer::local(text, cx));
                 multibuffer.push_excerpts(
                     buffer,
                     ranges.into_iter().map(|range| ExcerptRange {