Decouple reserved words and keywords

[sjbarag]

It's become obvious that keywords and reserved words receive separate treatment within the Reference BrightScript Implementation (RBI), e.g.:

1. The as keyword is not a reserved word, so is a valid variable name.
2. The Type reserved word is not a keyword - just a reserved identifier.

Since these properties seem to be able to vary independently, it makes sense to handle them separately within the lexer and parser created here.

• Reserved words should be lexed as regular identifiers. Handling of reserved word usage can be handled in one of two ways:
  • (Preferred, and probably simpler) Identifiers (see Token.ts) gain a new boolean member isReserved; the interpreter simply checks for truthiness of that value.
  • (Slower and likely involves lots of repeated logic) The interpreter checks all identifiers against the reserved word list at each use.
  • Example: type("foo") gets lexed into something like [ Identifier("type"), LefParen, Literal("foo"), RightParen ]
• Keywords are lexed into Tokens with the kind property equal to their own Lexemes.
  • Example: foo and bar gets lexed into something like [ Identifier("foo"), And, Identifier("bar") ]

[sjbarag]

Closed via #95 (I linked to the wrong issue, whoops)