Travis: Run Sublime Syntax Tests

[tajmone]

Add new Travis CI job to run Sublime Text syntax tests.

There should be different ways to do this:

1. Add the official syntax tester binary made available by Sublime Text developers (I vaguely remember that they made such binaries available for CI testing purposes, but I need to find references).
2. Use a dedicated package like UnitTesting, or the Sublime Text Syntax Tests GitHub action:
   • https://github.com/SublimeText/UnitTesting
   • https://github.com/marketplace/actions/sublime-text-syntax-tests

The UnitTesting solution might be too feature rich for the current goals of this project; also, these packages are now covering also the (so-called) "Sublime Text 4".