Add missing builtins

[AmjadHD]

• GC_enable
• GC_enableMarkAndSweep
• nimGC_setStackBottom
• nimThreadProcWrapperBody

[Araq]

Oh god nooooo, instead remove the existing builtins, it makes no sense to highlight them differently and it's super annoying. All these syntax highlighters that don't understand what Nim's keywords are. They are all listed in the manual, there are no others.

[AmjadHD]

You are right, I didn't notice that other syntaxes do not highlight builtins differently. Sublime Text syntaxes do. So God-willing I'll remove them from Github's grammars (tmLanguage) only.

[AmjadHD]

@Varriount

[AmjadHD]

@Varriount.