Add ISPC language support

[aneshlya]

Adds the ISPC language, full name "Intel® Implicit SPMD Program Compiler" https://github.com/ispc/ispc, extension .ispc, along with its header files .isph.
ISPC is a C-based language with extensions for SIMD programming.
ISPC is used across different game engines (e.g. Unreal Engine, Unity), production renderers (e.g Dreamworks' MoonRay) and ray tracing libraries (e.g. Intel(R) Ospray)

Closes #5042

• I am adding a new language.

  • The extension of the new language is used in hundreds of repositories on GitHub.com.

    • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.ispc - 8.7k
    • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.isph - 644

  • I have included a real-world usage sample for all extensions added in this PR:
    - samples/ISPC/VideoFunctions.ispc, GPL-3
    - samples/ISPC/volume.ispc, BSD3C
    - samples/ISPC/program_city.ispc, MIT

  • I have included a syntax highlighting grammar: [ispc.syntax]

  • I have added a color

    • Hex value: #2D68B1
    • Rationale: it is used in a header on official website: https://ispc.github.io/ispc.html

  • I have updated the heuristics to distinguish my language from others using the same extension. N/A. There are no languages with the same extension.

[lildude]

• https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.isph - 644

This low usage will block this PR. You might want to consider removing it. People can use an override for these files until usage is sufficient to add support.

[aneshlya]

This low usage will block this PR. You might want to consider removing it. People can use an override for these files until usage is sufficient to add support.

I removed .isph extension and added more .ispc samples.

[lildude]

LGTM. Thanks.

Important

The changes in this PR will not appear on GitHub until the next release has been made and deployed. See here for more details.