FR: Option to turn off completions

[chrisgrieser]

Hi, thanks for this great package!

I am using LSP-css and its completions together with the completions form this package result in unnecessary, duplicate completions.

I personally prefer the ones from LSP-css, so I'd like to have an option to turn the ones from CSS3 off. I could turn them off by deleting css3_completions.py, but having a setting would obviously better, e.g. to avoid having to delete the completions file every update.