Completion commit characters are too aggressive

Various things that shouldn't be commit characters are causing the editor to accept the completion. Some examples I've hit immediately:

Parameter name completion, hitting , or ) after a short identifier will cause the completion to be accepted.

I think this is just not implemented soft-select: there's 2 completion modes in Roslyn, hard select (when things like enter/punctuation should be considered commit characters) and soft select, when it shouldn't. I think you likely need to steal some of my code from O# around looking for suggestion mode:

https://github.com/OmniSharp/omnisharp-roslyn/blob/master/src/OmniSharp.Roslyn.CSharp/Services/Completion/CompletionService.cs#L134
https://github.com/OmniSharp/omnisharp-roslyn/blob/master/src/OmniSharp.Roslyn.CSharp/Services/Completion/CompletionListBuilder.cs#L98-L161
Enter after the { accepts the completion element.

This looks like completion being triggered in locations where it should not. We're probably going to need to write an accessor for this: https://github.com/OmniSharp/omnisharp-roslyn/blob/master/src/OmniSharp.Roslyn.CSharp/Services/Completion/CompletionService.cs#L88-L89.

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