Better handle the .mask extension

[lildude]

Description

As pointed out in #4641, the handling of the .mask extension can be improved as at the moment all Unity3D Asset masks, Mask (aka MaskJS) and Gentoo's portage mask files are all being classified as Mask aka MaskJS.

This PR addresses that by adding the extension to Unity3D Asset and the specific filenames to Text and then implementing an heuristic to specifically identify the Unity3D mask files. An heuristic isn't implemented for the other two languages as there is no consistent and reliable method of consistently identifying them as a particular language. It is also not necessary as we can explicitly identify the portage files by filename leaving everything else up to the classifier.

Checklist:

• I am fixing a misclassified language
  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • All Gentoo portage samples come from https://github.com/krizalys/sublime-portage
      • handFingers.mask comes from https://github.com/ValveSoftware/steamvr_unity_plugin/blob/87efb96e0f533391f78242d0dede9d597ef8cf49/Assets/SteamVR/Models/handFingers.mask - Sample license(s):
    • All files are covered by the BSD 3-Clause license.
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

Fixes #4641