wrap keyword matching code to functions

[gasolin]

current patch use XXX == 'keyword' || XXX == 'trans_keyword' inline to support different languages

could be replaced by a set of isKeyword[Keyword] function and wrap the matching detail inside of the function

ex, instead of XXX == 'Symbol' || XXX == '中文'

replace to isKeyword(XXX)

and define function

function isKeywordSymbol(value) {
  return value == 'Symbol' || value == '中文' 
}

The change could be proposed to TypeScript for maintainability.

By replacing the isKeyword[Keyword] functions, LingaScript could even support parse multi-native-language keywords in the same compiler

[gasolin]

first attempt at 3b007cc