Add common raytrace shader extensions to GLSL

[spnda]

Description

This adds the commonly used rmiss, and rchit shader extensions to GLSL. With Vulkan Ray Tracing getting much more popular, these are being used in a lot of software. They are officially used by NVIDIA and Khronos.
These extensions are also listed as the default extensions for ray tracing related shaders inside of glslang, which is part of the Vulkan SDK.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3Archit+NOT+nothack
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3Armiss+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • closesthit.rchit
      • miss.rmiss
    • Sample license(s): MIT
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

Thanks for this PR but unfortunately it looks like only .rchit and .rmiss meet our minimum usage requirement for inclusion right now; .rgen is close, but the others aren't anywhere near close:

.rahit:

Total files found: 975
Unique public user/repos: 51
Unique owners: 50

.rcall:

Total files found: 798
Unique public user/repos: 56
Unique owners: 54

.rchit:

Total files found: 2549
Unique public user/repos: 240
Unique owners: 203

.rgen:

Total files found: 3894
Unique public user/repos: 180
Unique owners: 165

.rint:

Total files found: 829
Unique public user/repos: 56
Unique owners: 54

.rmiss:

Total files found: 1643
Unique public user/repos: 237
Unique owners: 211

Please remove all but .rchit and .rmiss. These other extensions can be added as and when they reach sufficient usage levels.

I have not included a real-world sample as it is identical to the syntax of other GLSL files (they're just used with a extra GLSL extension that provides new structs and functions, which is the only difference), however if I have to, I'd happy to add something from the samples NVIDIA and Khronos provide (both are licensed under Apache-2.0).

Whilst the syntax may be the same, we will require samples if these files contain keywords/tokens that are unique to the files with these extensions when looked at in isolation, especially if they don't occur in any of our other GLSL samples. This is to train the classifier in the event another language starts using the same extensions.

If in doubt, always add a sample.

[lildude]

LGTM. Thanks.