Update grammar for unicode literals

docs/Solidity.g4

@@ -253,6 +253,7 @@ primaryExpression
   | numberLiteral
   | hexLiteral
   | stringLiteral
+  | unicodeStringLiteral
   | identifier ('[' ']')?
   | TypeKeyword
   | tupleExpression
@@ -461,6 +462,13 @@ StringLiteralFragment
   : '"' DoubleQuotedStringCharacter* '"'
   | '\'' SingleQuotedStringCharacter* '\'' ;
 
+unicodeStringLiteral
+  : UnicodeStringLiteralFragment+ ;
+
+UnicodeStringLiteralFragment
+  : 'unicode"' DoubleQuotedStringCharacter* '"'
+  | 'unicode\'' SingleQuotedStringCharacter* '\'' ;
+
 fragment
 DoubleQuotedStringCharacter
   : ~["\r\n\\] | ('\\' .) ;