Support better C++ annotation highlighting

* Uses some simple highlighting within annotations
* Supports multi-line annotations
  (see compiler-explorer/compiler-explorer#878)

src/cpp/cpp.test.ts

@@ -856,5 +856,55 @@ testTokenization('cpp', [
 				{ startIndex: 4, type: 'comment.cpp' }
 			]
 		}
+	],
+
+	[
+		{
+			line: '[[nodiscard]]',
+			tokens: [{ startIndex: 0, type: 'annotation.cpp' }]
+		}
+	],
+	[
+		{
+			// Example from http://eel.is/c++draft/dcl.attr
+			line: '[[using CC: opt(1), debug]]',
+			tokens: [
+				{ startIndex: 0, type: 'annotation.cpp' }, // [[
+				{ startIndex: 2, type: 'keyword.cpp' }, // using
+				{ startIndex: 7, type: '' },
+				{ startIndex: 8, type: 'annotation.cpp' }, // CC
+				{ startIndex: 10, type: 'delimiter.cpp' }, // colon
+				{ startIndex: 11, type: '' },
+				{ startIndex: 12, type: 'annotation.cpp' }, // opt
+				{ startIndex: 15, type: 'delimiter.parenthesis.cpp' }, // (
+				{ startIndex: 16, type: 'annotation.cpp' }, // 1
+				{ startIndex: 17, type: 'delimiter.parenthesis.cpp' }, // )
+				{ startIndex: 18, type: 'delimiter.cpp' }, // ,
+				{ startIndex: 19, type: '' },
+				{ startIndex: 20, type: 'annotation.cpp' } // debug]]
+			]
+		}
+	],
+	[
+		// Multiline and comments.
+		{
+			line: '[[nodiscard /*commented*/',
+			tokens: [
+				{ startIndex: 0, type: 'annotation.cpp' },
+				{ startIndex: 11, type: '' },
+				{ startIndex: 12, type: 'comment.cpp' }
+			]
+		},
+		{
+			line: ']] int i;',
+			tokens: [
+				{ startIndex: 0, type: 'annotation.cpp' },
+				{ startIndex: 2, type: '' },
+				{ startIndex: 3, type: 'keyword.int.cpp' },
+				{ startIndex: 6, type: '' },
+				{ startIndex: 7, type: 'identifier.cpp' },
+				{ startIndex: 8, type: 'delimiter.cpp' }
+			]
+		}
 	]
 ]);

src/cpp/cpp.ts

@@ -298,7 +298,7 @@ export const language = <languages.IMonarchLanguage>{
 			{ include: '@whitespace' },
 
 			// [[ attributes ]].
-			[/\[\[.*\]\]/, 'annotation'],
+			[/\[\[/, { token: 'annotation', next: '@annotation' }],
 
 			[/^\s*#include/, { token: 'keyword.directive.include', next: '@include' }],
 
@@ -384,6 +384,15 @@ export const language = <languages.IMonarchLanguage>{
 			[/.*/, 'string.raw']
 		],
 
+		annotation: [
+			{ include: '@whitespace' },
+			[/using|alignas/, 'keyword'],
+			[/[a-zA-Z0-9_]+/, 'annotation'],
+			[/[,:]/, 'delimiter'],
+			[/[()]/, '@brackets'],
+			[/\]\]/, { token: 'annotation', next: '@pop' }]
+		],
+
 		include: [
 			[
 				/(\s*)(<)([^<>]*)(>)/,