Remove `emit` keyword.

[Y-Less]

Issue description:

When asm was originally proposed there was some discussion here:

#180 (comment)

And the name __emit was instead agreed on as the best name - both for consistency with #emit and to reduce the chances of clashing with existing code by using the compiler-reserved prefix __. However, emit was also added as a keyword despite being a valid user-symbol.

Minimal complete verifiable example (MCVE):

main()
{
    new a = emit(ZERO.pri);
}

Workspace Information:

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

[Y-Less]

If a script wants this, it should be optionally enabled, not on by default:

#define emit __emit