Add resolveAdditionalTextEditsSupport capability to speed up completion

[rrudakov]

Based on this and this

[yyoncho]

This is not sufficient. We have to implement it in lsp-mode as well otherwise the additional text edits won't be applied.

cc @kiennq

[kiennq]

Let me make sure if I understand this correctly, we will allow resolveAdditionalTextEditsSupport, and will always resolve completion item when determine it from the popup?
Is there server capability corresponding to this so I can choose to resolve completion item or not?

[yyoncho]

thank you!

[rrudakov]

Thanks a lot!