Added missing js es2017 keywords

`async` and `await` are now part of javascript since es2017. `yield` was there even before that

examples:
````
//async and await:
function resolveAfter2Seconds(x) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function add1(x) {
  var a = resolveAfter2Seconds(20);
  var b = resolveAfter2Seconds(30);
  return x + await a + await b;
}

add1(10).then(v => {
  console.log(v);  // prints 60 after 2 seconds.
});

async function add2(x) {
  var a = await resolveAfter2Seconds(20);
  var b = await resolveAfter2Seconds(30);
  return x + a + b;
}

add2(10).then(v => {
  console.log(v);  // prints 60 after 4 seconds.
});
//yield:
function* foo() {
  var index = 0;
  while (index <= 2)
    yield index++;
}
````
This should close issue #2455 too.

References:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield

Closes #3041, fixes #2455

PowerEditor/src/langs.model.xml

@@ -125,10 +125,10 @@
         </Language>
 		<!-- Embedded javascript -->
         <Language name="javascript" ext="" commentLine="//" commentStart="/*" commentEnd="*/">
-            <Keywords name="instre1">abstract boolean break byte case catch char class const continue debugger default delete do double else enum export extends final finally float for from function goto if implements import in instanceof int interface let long native new null of package private protected public return short static super switch synchronized this throw throws transient try typeof var void volatile while with true false prototype</Keywords>
+            <Keywords name="instre1">abstract async await boolean break byte case catch char class const continue debugger default delete do double else enum export extends final finally float for from function goto if implements import in instanceof int interface let long native new null of package private protected public return short static super switch synchronized this throw throws transient try typeof var void volatile while with true false prototype yield</Keywords>
         </Language>
         <Language name="javascript.js" ext="js jsm jsx ts tsx" commentLine="//" commentStart="/*" commentEnd="*/">
-            <Keywords name="instre1">abstract boolean break byte case catch char class const continue debugger default delete do double else enum export extends final finally float for from function goto if implements import in instanceof int interface let long native new null of package private protected public return short static super switch synchronized this throw throws transient try typeof var void volatile while with true false prototype</Keywords>
+            <Keywords name="instre1">abstract async await boolean break byte case catch char class const continue debugger default delete do double else enum export extends final finally float for from function goto if implements import in instanceof int interface let long native new null of package private protected public return short static super switch synchronized this throw throws transient try typeof var void volatile while with true false prototype yield</Keywords>
 			<Keywords name="type1">Array Date eval hasOwnProperty Infinity isFinite isNaN isPrototypeOf Math NaN Number Object prototype String toString undefined valueOf</Keywords>
 			<Keywords name="instre2">alert all anchor anchors area assign blur button checkbox clearInterval clearTimeout clientInformation close closed confirm constructor crypto decodeURI decodeURIComponent defaultStatus document element elements embed embeds encodeURI encodeURIComponent escape event fileUpload focus form forms frame innerHeight innerWidth layer layers link location mimeTypes navigate navigator frames frameRate hidden history image images offscreenBuffering open opener option outerHeight outerWidth packages pageXOffset pageYOffset parent parseFloat parseInt password pkcs11 plugin prompt propertyIsEnum radio reset screenX screenY scroll secure select self setInterval setTimeout status submit taint text textarea top unescape untaint window</Keywords>
 			<Keywords name="type2">onblur onclick onerror onfocus onkeydown onkeypress onkeyup onmouseover onload onmouseup onmousedown onsubmit</Keywords>