Add support for const type parameters

Author: guillaumebrunerie

common/corpus/functions.txt

@@ -18,6 +18,10 @@ function foo<T, U>(this: T[]): U[] {
   return []
 }
 
+function foo<const T, const U extends string>(x: T, y: U) {
+
+}
+
 ---
 
 (program
@@ -57,7 +61,20 @@ function foo<T, U>(this: T[]): U[] {
         pattern: (this)
         type: (type_annotation (array_type (type_identifier)))))
     return_type: (type_annotation (array_type (type_identifier)))
-    body: (statement_block (return_statement (array)))))
+    body: (statement_block (return_statement (array))))
+  (function_declaration
+    name: (identifier)
+    type_parameters: (type_parameters
+      (type_parameter name: (type_identifier))
+      (type_parameter name: (type_identifier) constraint: (constraint (predefined_type))))
+    parameters: (formal_parameters
+      (required_parameter
+        pattern: (identifier)
+        type: (type_annotation (type_identifier)))
+      (required_parameter
+        pattern: (identifier)
+        type: (type_annotation (type_identifier))))
+    body: (statement_block)))
 
 ==================================
 New object with type arguments

common/define-grammar.js

@@ -894,6 +894,7 @@ module.exports = function defineGrammar(dialect) {
       ),
 
       type_parameter: $ => seq(
+        optional('const'),
         field('name', $._type_identifier),
         field('constraint', optional($.constraint)),
         field('value', optional($.default_type))