feat(cmp): use parsed snippet for cmp documentation

[folke]

This PR uses the parsed snippet as documentation instead of the raw snippet.

With this PR

Before

Edit: not the same snippet, but you get the idea :)

[garymjr]

@folke if you include utils.expand_vars in utils.preview we can catch all the variables in the preview. Otherwise the variables that nvim doesn't support are returned as empty strings.

[folke]

Good idea. Done!

[garymjr]

Thank you!