Update spec for new CompletionItem field

[MikhailArkhipov]

New 'preselect' field in the ls.CompletionItem.

Per microsoft/vscode-languageserver-node#371

and

microsoft/vscode#35551
microsoft/vscode@62e0f34

The feature added recently to VSC (1.25) allows completion provider 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.

[KamasamaK]

This was resolved by #515

[dbaeumer]

@KamasamaK correct. Closing the item.