Convert query capture indices to style ids

* Introduce a Theme struct as a new part of the app's settings
* Store on each Language a ThemeMap, which converts the capture ids
  from that language's highlight query into StyleIds, which identify
  styles in the current Theme.
* Update `highlighted_chunks` methods to provide StyleIds instead of
  capture ids.

zed/languages/rust/highlights.scm

@@ -1,6 +1,49 @@
+(type_identifier) @type
+
+(call_expression
+  function: [
+    (identifier) @function
+    (scoped_identifier
+      name: (identifier) @function)
+    (field_expression
+      field: (field_identifier) @function.method)
+  ])
+
+(field_identifier) @property
+
+(function_item
+  name: (identifier) @function.definition)
+
 [
+  "async"
+  "break"
+  "const"
+  "continue"
+  "dyn"
   "else"
+  "enum"
+  "for"
   "fn"
   "if"
+  "impl"
+  "let"
+  "loop"
+  "match"
+  "mod"
+  "move"
+  "pub"
+  "return"
+  "struct"
+  "trait"
+  "type"
+  "use"
+  "where"
   "while"
-] @keyword
+] @keyword
+
+(string_literal) @string
+
+[
+  (line_comment)
+  (block_comment)
+] @comment