Add support for C++26 Pack indexing

[aandrejevas]

Feature Request

IntelliSense does not recognize pack indexing syntax and because of that it shows error squiggles and also colorization of some statements is wrong after a pack indexing usage (in the example test_concept is colored incorrectly).

Please add support for pack indexing.

[sean-mcmanus]

@aandrejevas Currently, our C++26 parser support is very minimal (C++23 support isn't finished yet).