Add support to contribute completion items as extensions to jdt.ls

[gayanper]

JDT Core has most of the completion support which triggered as autocomplete on type or manual triggering. It would be nice to allow other extensions to provide completions in either of these scenarios.

One use case is to provide more expensive but smart completions based on receiver type of the expression like in IJ.

[gayanper]

@jdneo @rgrunber Do we have this support already in JDT.LS ?

[rgrunber]

Seems similar to #1731 & #613 .

[jdneo]

So far, we haven't exposed this capability as a contribution point. Now what we have is a ranking provider API allowing extensions to change the ranking value of the items.

If we want to allow extensions to contribute completion items, we might consider add new APIs here: https://github.com/eclipse/eclipse.jdt.ls/blob/master/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/contentassist/ICompletionContributionService.java