Add a setting for custom associations between languages and files (#9290)

Closes #5178

Release Notes:

- Added a `file_types` setting that can be used to associate languages
with file names and file extensions. For example, to interpret all `.c`
files as C++, and files called `MyLockFile` as TOML, add the following
to `settings.json`:

    ```json
    {
      "file_types": {
        "C++": ["c"],
        "TOML": ["MyLockFile"]
      }
    }
    ```

As with most zed settings, this can be configured on a per-directory
basis by including a local `.zed/settings.json` file in that directory.

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/editor/src/test/editor_test_context.rs

@@ -9,7 +9,7 @@ use gpui::{
 };
 use indoc::indoc;
 use itertools::Itertools;
-use language::{Buffer, BufferSnapshot};
+use language::{Buffer, BufferSnapshot, LanguageRegistry};
 use parking_lot::RwLock;
 use project::{FakeFs, Project};
 use std::{
@@ -120,6 +120,18 @@ impl EditorTestContext {
         })
     }
 
+    pub fn language_registry(&mut self) -> Arc<LanguageRegistry> {
+        self.editor(|editor, cx| {
+            editor
+                .project
+                .as_ref()
+                .unwrap()
+                .read(cx)
+                .languages()
+                .clone()
+        })
+    }
+
     pub fn update_buffer<F, T>(&mut self, update: F) -> T
     where
         F: FnOnce(&mut Buffer, &mut ModelContext<Buffer>) -> T,