[Java] Handle commands expected by eclipse.java.lsp

[svenefftinge]

The Java language server seems to expect a couple of commands to be available on the client.
For example, on first start when the classpath is incomplete, we see a message with three options.

Clicking e.g. on the first results in the following error message:

The command 'java.ignoreIncompleteClasspath.help' cannot be executed. There are no active handlers available for the command.

[tsmaeder]

See also #4050. To fix this for pure Theia language server extensions, thought, I guess it would have to be addressed in monaco-languageclient.