Stop gathering completions when completions are added from LSP

[SCWells72]

In most JetBrains IDEs, when context-specific completions are added, no completions are added from other contributors, specifically the WordCompletionContributor which adds words found in the same document. Right now that's not happening for the LSPCompletionContributor, so you get completion lists like:

Instead LSPCompletionContributor should register itself before WordCompletionContributor and, if LSP-based competions were added, should stop collecting completions, e.g.:

If the user explicitly types the completion trigger more than once, though, additional completions can/should be added to the list:

[SCWells72]

See #639.