Add outline support for Ruby singleton methods (#2592)

This pull request add support for Ruby singleton methods in the document
outline.

**Before**

<img width="1490" alt="outline-before"
src="https://github.com/zed-industries/zed/assets/503025/e90afd4c-bcdd-477b-92cf-fe34cec54980">

**After**

<img width="1493" alt="outline-after"
src="https://github.com/zed-industries/zed/assets/503025/067cc80b-7718-4395-9079-4677a689b9be">

Release Notes:

- Added support for singleton methods in Ruby outline view. Fixed
[#1442](https://github.com/zed-industries/community/issues/1442).

crates/zed/src/languages/ruby/outline.scm

@@ -6,6 +6,12 @@
     "def" @context
     name: (_) @name) @item
 
+(singleton_method
+    "def" @context
+    object: (_) @context
+    "." @context
+    name: (_) @name) @item
+
 (module
     "module" @context
     name: (_) @name) @item