Add highlighting and correct file extensions for TI Program

[TIny-Hacker]

This PR adds highlighting and updates file extensions for the already existing TI Program language in linguist, which was added in #3258.

Description

I've created and added a syntax highlighting grammar for all tokens in the TI-BASIC programming language, which is detected as TI Program by linguist. I've chosen not to change the name TI Program to TI-BASIC as the are a few community-created languages for the TI-83 / 84 Plus family which also fall under the TI Program label, as mentioned in #6295. This grammar applies to those languages as well, as they re-use the same tokens as TI-BASIC, meaning that they can and should still all fall under the TI Program label.

I've also removed the .8xk and .8xk.txt extension and replaced it with .83p and .83p.txt, as discussed in #6295. The .8xk extension and variants are only used by binary files, not source, meaning that it does not make sense to include them in linguist.

I've also added samples for .8xp, which currently has no samples.

• https://github.com/TIny-Hacker/Cow-Clicker-84-Plus/blob/master/cowsource.8xp (MIT license)
• https://github.com/TIny-Hacker/celtic-basic-demos/blob/main/CIRCLES.8xp (MIT license)
• https://github.com/TIny-Hacker/celtic-basic-demos/blob/main/CLOCK.8xp (MIT license)

Checklist:

• I am changing the source of a syntax highlighting grammar
  • Old: No highlighting.
  • New: https://github.com/TIny-Hacker/language-ti-basic

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.