Remove `*begin`, `*end` and `*then`

[Daniel-Cortez]

Issue description:

• These keywords don't have any unique meaning; they only duplicate the functionality of the existing C-style curly bracket syntax.
• They can complicate diagnostics, causing counter-intuitive error messages, as shown in #512.
• They were already removed in Pawn 3.3, so we could do the same in this fork.

Minimal complete verifiable example (MCVE):

Workspace Information:

• Compiler version: 3.10.10
• Command line arguments provided (or sampctl version):
• Operating System:

[Daniel-Cortez]

@Y-Less Why did you close this issue if the PR that's supposed to fix it is still not merged? EDIT: I see, it seems that there's a build error in that PR (and I honestly don't remember having in on my PC); I'll see if I can fix it this weekend.