cpp Lexer: Updates for Standard Library User-Defined Literals (chrono, complex)

[swheaton]

1. Add year (y) and date (d) std::chrono literals (introduced in C++20) - https://en.cppreference.com/w/cpp/symbol_index/chrono_literals
2. Add std::complex literals (i, il, if) - https://en.cppreference.com/w/cpp/symbol_index/complex_literals
3. Fix to allow digit separators in std::chrono and std::complex literals

[dblessing]

@swheaton Thanks for your contribution. I left one item for your consideration.

[swheaton]

Thanks for review!