[3.16] [Completion] Support insertTextModeSupport

[kiennq]

Please refer to microsoft/language-server-protocol#83, #1939 and https://github.com/microsoft/language-server-protocol/blob/gh-pages/_specifications/specification-3-16.md#textDocument_completion.

This capability replaces keepWhitespace

[yyoncho]

@kiennq wasn't that fixed?

[kiennq]

Yes, it's fixed.

[kiennq]

Reopen this, I was wrong, this is insertTextModeSupport, not insertReplaceSupport