Generated ANTLR files

[pchaigno]

Just a reminder for myself to do it later. It's also a good first issue for beginners with Linguist.

From #4404:

Many of these files seem to be generated: https://github.com/search?q=extension%3Ag+%22generated+by+xtext%22&type=Code. Maybe we should add that to the generated.rb file? (I can do it in a separate pull request if you prefer.)

The solution is to add the appropriate code in lib/linguist/generated.rb to recognize these files as generated.

[stale]

This issue has been automatically marked as stale because it has not had activity in a long time. If this issue is still relevant and should remain open, please reply with a short explanation (e.g. "I have checked the code and this issue is still relevant because ___."). Thank you for your contributions.

[Nixinova]

Should this still be done?

[Nixinova]

Added in #5199, can be closed