Add support for UTF-8 literals

Author: damieng

corpus/literals.txt

@@ -314,6 +314,50 @@ class A {
           (identifier)
           (equals_value_clause (string_literal (escape_sequence)))))))))
 
+=======================================
+utf-8 string literals
+=======================================
+
+var a = "lower"u8;
+var b = "upper"U8;
+var c = @"lower"u8;
+var d = @"upper"U8;
+---
+
+ (compilation_unit
+    (global_statement
+      (local_declaration_statement
+        (variable_declaration
+          (implicit_type)
+          (variable_declarator
+            (identifier)
+            (equals_value_clause
+              (string_literal))))))
+    (global_statement
+      (local_declaration_statement
+        (variable_declaration
+          (implicit_type)
+          (variable_declarator
+            (identifier)
+            (equals_value_clause
+              (string_literal))))))
+    (global_statement
+      (local_declaration_statement
+        (variable_declaration
+          (implicit_type)
+          (variable_declarator
+            (identifier)
+            (equals_value_clause
+              (verbatim_string_literal))))))
+    (global_statement
+      (local_declaration_statement
+        (variable_declaration
+          (implicit_type)
+          (variable_declarator
+            (identifier)
+            (equals_value_clause
+              (verbatim_string_literal)))))))
+
 ==================================================
 string literals containing newline
 ==================================================

grammar.js

@@ -1644,7 +1644,7 @@ module.exports = grammar({
         $._string_literal_fragment,
         $.escape_sequence
       )),
-      '"'
+      choice('"', '"U8', '"u8')
     ),
 
     _string_literal_fragment: $ => token.immediate(prec(1, /[^"\\\n]+/)),
@@ -1655,7 +1655,7 @@ module.exports = grammar({
         /[^"]/,
         '""',
       )),
-      '"'
+      choice('"', '"U8', '"u8')
     )),
 
     // Comments

src/grammar.json

@@ -9214,8 +9214,21 @@
           }
         },
         {
-          "type": "STRING",
-          "value": "\""
+          "type": "CHOICE",
+          "members": [
+            {
+              "type": "STRING",
+              "value": "\""
+            },
+            {
+              "type": "STRING",
+              "value": "\"U8"
+            },
+            {
+              "type": "STRING",
+              "value": "\"u8"
+            }
+          ]
         }
       ]
     },
@@ -9256,8 +9269,21 @@
             }
           },
           {
-            "type": "STRING",
-            "value": "\""
+            "type": "CHOICE",
+            "members": [
+              {
+                "type": "STRING",
+                "value": "\""
+              },
+              {
+                "type": "STRING",
+                "value": "\"U8"
+              },
+              {
+                "type": "STRING",
+                "value": "\"u8"
+              }
+            ]
           }
         ]
       }

src/node-types.json

@@ -6269,6 +6269,14 @@
     "type": "\"\"",
     "named": false
   },
+  {
+    "type": "\"U8",
+    "named": false
+  },
+  {
+    "type": "\"u8",
+    "named": false
+  },
   {
     "type": "$\"",
     "named": false