LSP: provide sortText in completion items

[Khady]

This PR adds fills the sortText field of completion items in textDocument/completion response. This is necessary to make sure that the client will respect the order decided by merlin.

Tested on emacs only.

https://microsoft.github.io/language-server-protocol/specification#textDocument_completion
tigersoldier/company-lsp#56 (comment)

Per specification, the server can specify sortText for each candidate. Candidates should be sorted by sortText if the server populates them, or sorted by the labels if sortText is not populated.

[andreypopp]

Thanks!