Lexer: Make VerbatimStringChar a fragment

VerbatimStringChar is not a token so turn it into a fragment.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Parser: First take types, expressions, and patterns

Most of the grammar is disabled but it is good enough for expressions
such as:

  let x = "hello"

Signed-off-by: Pekka Enberg <penberg@kernel.org>