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/zed/src/languages/rust.rs

@@ -474,7 +474,7 @@ mod tests {
         let language = crate::languages::language("rust", tree_sitter_rust::language(), None).await;
 
         cx.add_model(|cx| {
-            let mut buffer = Buffer::new(0, "", cx).with_language(language, cx);
+            let mut buffer = Buffer::new(0, cx.model_id() as u64, "").with_language(language, cx);
 
             // indent between braces
             buffer.set_text("fn a() {}", cx);