Poor error message for unicode escapes in keywords

Replacing the a in var with a unicode escape of ASCII a (\u0061) yields v\u0061r in place of this keyword.

## Source
print((function () { v\u0061r a = 1; return a; })())

┌─────────────┬──────────────────────────────────────────────────────────┐
│ d8          │                                                          │
│ node        │ SyntaxError: Keyword must not contain escaped characters │
├─────────────┼──────────────────────────────────────────────────────────┤
│ sm          │                                                          │
│             │ SyntaxError: var is an invalid identifier:               │
├─────────────┼──────────────────────────────────────────────────────────┤
│ node-ch     │                                                          │
│ ch-2.0      │ SyntaxError: Syntax error                                │
│ ch-master   │                                                          │
└─────────────┴──────────────────────────────────────────────────────────┘

IMO: SM's error is confusing, and our current error is completely useless. v8 has a good error here.

/cc @curtisman @bterlson

---

Some info to help investigation (line numbers as of commit 3656c42cd750fcd7883e10159a919309abed0046):

With the above repro, try putting a breakpoint in lib\Parser\kwd-swtch.h line 395 (code follows):

     case 'v':
        if (identifyKwds)

Step through from here and find out how the error is decided, and then determine how we would emit a better error message without hurting performance.

It appears we ultimately return JsErrorScriptCompile from lib\Jsrt\Jsrt.cpp line 2993:

        if (scriptFunction == nullptr)
        {
            PERFORM_JSRT_TTD_RECORD_ACTION_NOT_IMPLEMENTED(scriptContext);

            HandleScriptCompileError(scriptContext, &se);
            return JsErrorScriptCompile;
        }

Basically we know at that point that the script failed to compile, and that's all the information we have. We would need to return a more specific JsError to improve the error message.