Refactor keyword and internal/lexer to accomodate more complicated grammars

[mcy]

This PR contains various interdependent changes to the lexing framework:

1. token.Punct is now token.Keyword, which indicates a hard keyword, i.e. a keyword that cannot be an identifier in the grammar.
2. lexer.Lexer now distinguishes hard and soft keywords.
3. Popular keywords (for, if, else, and operators like :=) are now available in the lexer.
4. Newlines are always their own tokens in the lexer; they can be promoted to keywords with an impact on the grammar via a configurable callback, which can then be used to implement newline-terminated statements (like in Go).
5. Various diagnostics and helpers from parser have been factored out into internal packages for re-use.