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);
	}