Highlight instance vars, method defs, and keyword params distinctly

These three constructs are folded into broader shared captures, so themes
cannot color them independently:

- instance variables share `@variable.special` with `self`, `super`, and
  class variables
- method definitions share `@function.method` with method calls
- keyword parameters share `@variable.parameter` with positional parameters

Give each its own dotted sub-scope, following the sub-scoping convention
already used throughout this file (e.g. `function.method.builtin`,
`keyword.control.conditional`, `string.special.symbol`):

- (instance_variable)        -> @variable.special.instance
- method / singleton_method  -> @function.method.definition
- keyword_parameter          -> @variable.parameter.keyword

Fully backwards-compatible: Zed resolves highlight names by longest prefix,
so themes that only define `variable.special`, `function.method`, or
`variable.parameter` render these exactly as before. Themes can now opt in to
distinguishing them.

Author: orthodoX

languages/ruby/highlights.scm

@@ -81,13 +81,13 @@
   name: [
     (identifier)
     (constant)
-  ] @function.method)
+  ] @function.method.definition)
 
 (singleton_method
   name: [
     (identifier)
     (constant)
-  ] @function.method)
+  ] @function.method.definition)
 
 (method_parameters
   [
@@ -98,7 +98,7 @@
       [
         name: (identifier)
         ":"
-      ] @variable.parameter)
+      ] @variable.parameter.keyword)
   ])
 
 (block_parameters
@@ -138,10 +138,9 @@
 
 (super) @variable.special
 
-[
-  (class_variable)
-  (instance_variable)
-] @variable.special
+(class_variable) @variable.special
+
+(instance_variable) @variable.special.instance
 
 ((call
   !receiver