Detailed changes:
1) Implement a lexer for string literals that divides the string in format specifier `{}` including the format specifier modifier.
2) Adapt syntax highlighting to add ranges for the detected sequences.
3) Add a test case for the format string syntax highlighting.
- Adds a new AstElement trait that is implemented by all generated
node, token and enum structs
- Overhauls the code generators to code-generate all tokens, and
also enhances enums to support including tokens, node, and nested
enums
This wasn't a right decision in the first place, the feature flag was
broken in the last rustfmt release, and syntax highlighting of imports
is more important anyway
