Gitpod: Add Xtend support

[kuniss]

Please!

[kuniss]

You may check adding Xtend highlighting support, I created as VS code extension: https://marketplace.visualstudio.com/items?itemName=Grammarcraft.xtend-lang
:-)

[kuniss]

@svenefftinge, @akosyakov, you may update to version 0.4.0. It now comes with an embedded, simple Xtext LS which allows better grammar file-internal navigation, hovering and so on.

However, external reference resolving is not supported. To avoid an overwhelming error message flood due to that, the linking diagnostic is disabled (by configuration).
Nevertheless, worth to be integrated due to the other features, I think.

[svenefftinge]

Cool you keep working on it. We hope we can publish a first version with Vs code extension support pretty soon. So you can always update to the latest version yourself.