lex: support special characters and unicode

The lexer should support both ascii special (escaped) characters in strings and characters, and in general unicode support for strings and potentially identifiers. We need to investigate what other modern languages (zig, rust, hare) do for this - I know hare is quite unicode friendly. We aren't looking for general unicode "understanding" in femto with runes, etc right now, but the lexer shouldn't reject internationally written code.

This should also be fuzzed, pending #46