Add preselect field to CompletionItem

[MikhailArkhipov]

Fixes #515

VS Code 1.25 adds new preselect field to the CompletionItem. The field allows completion providers to specify which item should be initially selected in the completion list.

By default clients usually select first or last used items, which is limiting. A completion provider may choose to select different item based on other conditions, such as current code editor context, probability of use of certain items, and so on.

This request is to add equivalent field to the CompletionItem structure therefore allowing use of preselection in VS Code from language servers as it is possible from the regular completion provider.

The respective Node LS issue is microsoft/vscode-languageserver-node#371

[dbaeumer]

A first implementation can be found here

https://github.com/Microsoft/vscode-languageserver-node/tree/dbaeumer/371

[dbaeumer]

Closing. Got added to 3.9.0.

[MikhailArkhipov]

@dbaeumer - do we need to update specification to reflect the change?

[dbaeumer]

I did update the specification put forgot to push :-(