This commit adds a function that tries to determine the syntax highlighting class of NAME_REFs based on the usage.
It is used for highlighting injections (such as highlighting of doctests) as the semantic logic will most of the time result in unresolved references.
It also adds a color to unresolved references in HTML encoding.
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.
