Support Kotlin,Groovy in Gradle projects

[snjeza]

Fixes#4312
Requires eclipse-jdtls/eclipse.jdt.ls#3682

cc @fbricon@rgrunber@tsmaeder

[fbricon]

It should be advertised as "partial" support, since we don't provide completion, or pretty much any LSP smartness besides making sure the other java classes compile.

We should probably enable those by default.

[snjeza]

It should be advertised as "partial" support, since we don't provide completion, or pretty much any LSP smartness besides making sure the other java classes compile.

I have marked the properties as Experimental.

We should probably enable those by default.

Done.

[fbricon]

@snjeza thanks!