language: Make Buffer::new take an explicit ID (#2900)

See Linear description for the full explanation of the issue. This PR is
mostly a mechanical change, except for the one case where we do pass in
an explicit `next_id` instead of `model_id` in project.rs.

Release Notes:
- Fixed a bug where some results were not reported in project search in
presence of unnamed buffers.

crates/editor/src/movement.rs

@@ -756,7 +756,8 @@ mod tests {
             .select_font(family_id, &Default::default())
             .unwrap();
 
-        let buffer = cx.add_model(|cx| Buffer::new(0, "abc\ndefg\nhijkl\nmn", cx));
+        let buffer =
+            cx.add_model(|cx| Buffer::new(0, cx.model_id() as u64, "abc\ndefg\nhijkl\nmn"));
         let multibuffer = cx.add_model(|cx| {
             let mut multibuffer = MultiBuffer::new(0);
             multibuffer.push_excerpts(