editor: Update insert_text_format based on resolved completion (#13041)

Fixes #12920

VTSLS does not mark snippet completions as such in the initial
completion response - not until we resolve them; however, we do not
touch initial contents of completion during resolution, which led to us
not treating a snippet as such.

Release Notes:

- Fixed snippet completions sometimes being treated as plain text
completions when using VTSLS