Improve handling of __dir__ and fixed list of GAP globals

[embray]

This is basically the same as https://trac.sagemath.org/ticket/27923

Sage's libgap did this because it tried to restrict the tab-completion to only "documented" functions in GAP, but where "documented" is somewhat ill-defined, and generating the complete list of documented globals at runtime is very slow.

I have asked the GAP developers about this in the past and they have also recommended against it. Using a hard-coded list also means globals added by common packages are not picked up.

Generating a list of global variables (excluding GAP keywords) is simple and reasonably fast:

In [23]: keywords = set(gap.GAPInfo['Keywords'])

In [24]: gap_globals = [g for g in gap.Filtered(gap.NamesGVars(), gap.IsBoundGlobal) if g not in keywords]

In [25]: %timeit gap_globals = [g for g in gap.Filtered(gap.NamesGVars(), gap.IsBoundGlobal) if g not in keywords]
51.8 ms ± 1.61 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)