Fixed tracking of argument scope.

[xeioex]

This properly catches unsupported "await" in arguments.

This fixes #730 issue on Github.

Checklist

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• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
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[VadimZhestikov]

May be we can use more simple approach instead of js_parser_scope_begin/end, like
just increment/decrement parser->scope->in_args instead of assigning it 1 or 0