Add .txx extension to C++

[wroyca]

Include support for the ".txx" file extension, which has been commonly used for C++ template implementation across various build systems. build2/bdep#5

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • .txx - https://github.com/search?q=path%3A*.txx&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/build2/build2/blob/a1a01cbce316c5da7aa94e6b6a71eae98bffed1c/libbuild2/target.txx#L4
      • https://github.com/Kitware/VTK/blob/6d5a55c728ee132a818da88ad48af899c9baa6ec/Common/Core/vtkSparseArray.txx#L4
      • https://github.com/hsundar/dendro/blob/f3337fc933ffaca885bf159548dfb224dbac5cbe/include/par/search.txx#L4
    • Sample license(s): MIT, 3-BSD, GPL2 respectively
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

Please add one to three of the most representative samples you have referenced to the C++ samples directory… we need the actual files to train the classifier, not just the links 😉

[wroyca]

Please add one to three of the most representative samples you have referenced to the C++ samples directory… we need the actual files to train the classifier, not just the links wink

Oops, done. Thank! 🚀

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.