[RFC] Drop xtext-web and gwt support

[szarnekow]

The web support still depends on very outdated technology (e.g., an old version of Ace Editor), and the GWT support is not actively used by the framework's maintainers; thus, it's very hard to assess its quality and ensure it's still operational.

We plan to drop xtext-web and the xbase-gwt support in one of the upcoming releases, likely immediately in 2.43