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/search/src/buffer_search.rs

@@ -1141,7 +1141,7 @@ mod tests {
     use super::*;
     use editor::{DisplayPoint, Editor};
     use gpui::{Context, Hsla, TestAppContext, VisualTestContext};
-    use language::{Buffer, BufferId};
+    use language::Buffer;
     use smol::stream::StreamExt as _;
     use unindent::Unindent as _;
 
@@ -1161,9 +1161,7 @@ mod tests {
     ) -> (View<Editor>, View<BufferSearchBar>, &mut VisualTestContext) {
         init_globals(cx);
         let buffer = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
+            Buffer::local(
                 r#"
                 A regular expression (shortened as regex or regexp;[1] also referred to as
                 rational expression[2][3]) is a sequence of characters that specifies a search
@@ -1171,6 +1169,7 @@ mod tests {
                 for "find" or "find and replace" operations on strings, or for input validation.
                 "#
                 .unindent(),
+                cx,
             )
         });
         let cx = cx.add_empty_window();
@@ -1519,13 +1518,7 @@ mod tests {
             expected_query_matches_count > 1,
             "Should pick a query with multiple results"
         );
-        let buffer = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                buffer_text,
-            )
-        });
+        let buffer = cx.new_model(|cx| Buffer::local(buffer_text, cx));
         let window = cx.add_window(|_| gpui::Empty);
 
         let editor = window.build_view(cx, |cx| Editor::for_buffer(buffer.clone(), None, cx));
@@ -1721,13 +1714,7 @@ mod tests {
         for "find" or "find and replace" operations on strings, or for input validation.
         "#
         .unindent();
-        let buffer = cx.new_model(|cx| {
-            Buffer::new(
-                0,
-                BufferId::new(cx.entity_id().as_u64()).unwrap(),
-                buffer_text,
-            )
-        });
+        let buffer = cx.new_model(|cx| Buffer::local(buffer_text, cx));
         let cx = cx.add_empty_window();
 
         let editor = cx.new_view(|cx| Editor::for_buffer(buffer.clone(), None, cx));