Allow merging of custom and default completion items

[qsc-jhndnn]

My unsuccessful searching to solve this makes me think this needs to be a feature request - I would like the ability to merge the automatically generated completion items with my suggested completion items. I have a sample here - https://github.com/jhndnnqsc/monaco-test/blob/main/src/App.js. In this example, if the user hits ctrl+space provideCompletionItems returns an empty array and the automatic list is shown.

If the user hits the '.' character a suggestion list is returned - in this case only the suggested items are included

It seems like it would be useful to be able to automatically combine the two lists - if there is a method for that already please let me know.

[rcjsuen]

@alexdima This looks like a duplicate of #1850 to me.