enh(js) add the using keyword (#4135)

* Add the `using` keyword per the "Explicit Resource Management" proposal

This ECMAScript proposal adds a new keyword (`using`). It's reached
stage 3, which is "recommended for implementation"

Proposal: https://github.com/tc39/proposal-explicit-resource-management

ECMAScript proposal process: https://tc39.es/process-document/

Author: sufianrhazi

src/languages/lib/ecmascript.js

@@ -40,7 +40,9 @@ const KEYWORDS = [
   "import",
   "from",
   "export",
-  "extends"
+  "extends",
+  // It's reached stage 3, which is "recommended for implementation":
+  "using"
 ];
 const LITERALS = [
   "true",

test/markup/javascript/keywords.expect.txt

@@ -10,4 +10,5 @@
     <span class="hljs-keyword">if</span> (<span class="hljs-title function_">checkCondition</span>(classes[i]) === <span class="hljs-literal">undefined</span>)
       <span class="hljs-keyword">return</span> <span class="hljs-regexp">/\d+[\s/]/g</span>;
   }
+  <span class="hljs-keyword">using</span> val = <span class="hljs-title function_">condition</span>();
 }

test/markup/javascript/keywords.txt

@@ -10,4 +10,5 @@ function $initHighlight(block, cls) {
     if (checkCondition(classes[i]) === undefined)
       return /\d+[\s/]/g;
   }
+  using val = condition();
 }