Heuristic to disambiguate GAP and GDScript

[anantn]

GDScript files (.gd) are often misclassified as GAP code on Github.

Description

This rule updates classification of .gd files as either GAP or GDScript. Currently, several GDScript files are misclassified as GAP on Github (example search: https://github.com/search?q=language%3AGAP+game&type=Repositories)

Checklist:

• I am fixing a misclassified language
  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[pchaigno]

Thanks for your contribution!

[pchaigno]

@anantn Could you add a test for these changes in test_heuristics.rb?

[pchaigno]

LGTM!

Pinging @fingolfin, who added support for both GAP and GDScript in Linguist (#1046 and #1529), in case they want to review.

[anantn]

If the heuristic doesn't match I think we should still classify the script as GAP since we would try other strategies as @pchaigno mentioned. But no harm making the heuristic more robust!

Thanks to @fingolfin and @pchaigno for your help with this patch 👍

[anantn]

@pchaigno Could we land this now? 😄

[lildude]

LGTM.