perf(lex): use linear search over regex

[JohelEGP]

Resolves #513.

For the following table:

• Cppfront is built with the respective Cpp1 compiler configuration.
• LLVM is shorthand for Clang and Libc++.

Legend:

• main: The regex is an automatic object (branch main).
• find_if: The keywords are stored in static vector and queried with std::find_if.

mixed-as-for-variant-20-types	LLVM 17, Debug	LLVM 17, Release	GCC 13, Debug	GCC 13, Release
Cppfront time (main)	11.28 s	0.94 s	3.44 s	0.45 s
Cppfront time (find_if)	0.08 s	0.01 s	0.08 s	0.01 s

#513 minimal reproducer	LLVM 17, Debug	LLVM 17, Release	GCC 13, Debug	GCC 13, Release
Cppfront time (main)	14.30 s	1.06 s	2.53 s	0.34 s
Cppfront time (find_if)	0.03 s	0.02 s	0.04 s	0.01 s

reflect.h	LLVM 17, Debug	LLVM 17, Release	GCC 13, Debug	GCC 13, Release
Cppfront time (main)	13.19 s	1.27 s	6.03 s	0.75 s
Cppfront time (find_if)	0.19 s	0.02 s	0.19 s	0.01 s

Testing summary:

100% tests passed, 0 tests failed out of 660

Total Test time (real) =  96.59 sec

Acknowledgements:

• Tested thanks to https://github.com/modern-cmake/cppfront.
• The commit message uses Conventional Commits 1.0.0.