[python] Add highlighting for formatted string literals

[eisenwave]

https://docs.python.org/3/reference/lexical_analysis.html#formatted-string-literals

#112 did not yet add processing of formatted string literals. This results in certain wrong highlights like:

f"outer{"inner"}outer"

Since the highlighter is unaware of f-string specifics, it would highlight f"outer{" as if that was the whole string.