Enable LSP client snippet capabilities when snippet integration is configured

[clason]

Neovim just merged a PR (https://github.com/neovim/neovim/pull/13183) that sets capabilities.textDocument.completion.completionItem.snippetSupport = false by default.

Now users have to remember to enable it themselves; it would be convenient if completion-nvim set that capability automatically when integration of one of the supported snippet plugins is configured.

@runiq

[smartding]

this only affects snippets from lsp server, right? no need to change anything if I'm using ultisnips.

[clason]

this only affects snippets from lsp server, right? no need to change anything if I'm using ultisnips.

Yes.

[haorenW1025]

This could be nice, I'll implement this when I have some spare time(not now, unfortunately:cry:)