[TypeScript] Optimize labeled tuple members

This commit ...

1. optimizes implementation to speed it up by about 15%
2. rescopes labels to `variable.other.member` as variable scope is what C# uses
   and this is what labels are for. Create named access to tuple members as if
   they were normal struct members.

Note:
1. `meta.mapping.key` should be used in mappings only.
2. We shouldn't mix up sequence and mapping scopes, although a mapping is just
   a sequence of key-value pairs.

JavaScript/TypeScript.sublime-syntax

@@ -935,26 +935,27 @@ contexts:
             - ts-type-tuple-possible-member-label
 
   ts-type-tuple-possible-member-label:
-    - match: (?={{identifier_name}})
+    - match: ''
       branch_point: ts-type-tuple-member-label
       branch:
         - ts-type-tuple-member-label
         - immediately-pop
       pop: 1
-    - include: else-pop
 
   ts-type-tuple-member-label:
-    - match: ''
+    - match: '{{identifier_name}}'
+      scope: variable.other.member.js
       set:
-        - - match: ':'
-            scope: punctuation.separator.type.js
-            pop: 1
-          - match: (?=\S)
-            fail: ts-type-tuple-member-label
+        - ts-type-tuple-member-label-separator
         - ts-type-annotation-optional
-        - - match: '{{identifier_name}}'
-            scope: meta.mapping.key.js
-            pop: 1
+    - include: else-pop
+
+  ts-type-tuple-member-label-separator:
+    - match: ':'
+      scope: punctuation.separator.type.js
+      pop: 1
+    - match: (?=\S)
+      fail: ts-type-tuple-member-label
 
   ts-type-tuple-spread:
     - match: \.\.\.

JavaScript/tests/syntax_test_typescript.ts

@@ -860,11 +860,11 @@ let x: [ first: any, rest: ...any ];
 //    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ meta.type
 //     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ meta.sequence
 //     ^ punctuation.section.sequence.begin
-//       ^^^^^ meta.mapping.key
+//       ^^^^^ variable.other.member
 //            ^ punctuation.separator.type
 //              ^^^ support.type.any
 //                 ^ punctuation.separator.comma
-//                   ^^^^ meta.mapping.key
+//                   ^^^^ variable.other.member
 //                       ^ punctuation.separator.type
 //                         ^^^ keyword.operator.spread
 //                            ^^^ support.type.any
@@ -873,7 +873,7 @@ let x: [ first: any, rest: ...any ];
 
 let x: [
     typeof
-//  ^^^^^ meta.mapping.key
+//  ^^^^^ variable.other.member
     ?
 //  ^ storage.modifier.optional
     :