[JavaScript] Support for the nullish coalescing assignment (??=) operator (#3830)

Author: meraedit

javascript/javascript/JavaScriptLexer.g4

@@ -96,6 +96,7 @@ BitAndAssign:                   '&=';
 BitXorAssign:                   '^=';
 BitOrAssign:                    '|=';
 PowerAssign:                    '**=';
+NullishCoalescingAssign:        '??=';
 ARROW:                          '=>';
 
 /// Null Literals

javascript/javascript/JavaScriptParser.g4

@@ -444,6 +444,7 @@ assignmentOperator
     | '^='
     | '|='
     | '**='
+    | '??='
     ;
 
 literal

javascript/javascript/examples/Misc.js

@@ -37,4 +37,7 @@ var g = [,,]; // [undefined,undefined]
 function f([asdfg]=12345){}
 /* TODO:
 function async (proc, ...params) {}
-var async = 1;*/
+var async = 1;*/
+
+const a = { duration: 50 };
+a.duration ??= 10;