Add support for interface and abstract method implementation code lens

[hopehadfield]

Fixes redhat-developer/vscode-java#3813

Requires redhat-developer/vscode-java#3876

Before:

After:

[rgrunber]

So this just works because the underlying code supported it all along ?! Just widening a few parameters..

eclipse.jdt.ls/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/ImplementationCollector.java

Lines 120 to 124 in 806786e

	if (javaElement instanceof IMethod) {
	implementations = findMethodImplementations(monitor);
	} else if (javaElement instanceof IType) {
	implementations = findTypeImplementations(monitor);
	}

[rgrunber]

Overall, this looks good and works well. The only thing I have to consider is whether we care to be backwards compatible in the language server also 😜

Also, I found the implementation behaviour even for types (so independent of this PR) diverges from how Eclipse does it. For example, Eclipse will show as implementations, methods/types that override/extend non-interface, non-abstract classes. However, because of that isInterface / isAbstract logic prior to calling getCodeLens(..) we completely avoid rendering such classes. Might need to look back and see if there's a good reason we did this. If not, we could fix it in a later PR.

[rgrunber]

... The only thing I have to consider is whether we care to be backwards compatible in the language server also 😜

I've decided it's not worth doing this on the language server side. It's probably enough if we just mention the rename in the eventual changelog.

[rgrunber]

Looks good. Can you just remove those extraneous whitespaces (tabs) on the new lines in the 3 test classes. Ext, ITest & Test ? git show displays them.