Choose whether note is resolvable

## Feature Description

While working on #417, I've noticed that notes in `gitlab.nvim` are created as resolvable (i.e., as threads) when they are created directly with `require("gitlab").create_note()`, but as non-resolvable when they are first created as a draft note and then published. While I don't think this causes any real problems to users, it may be a little confusing why some notes need to be resolved and some not.

To match the behaviour of Gitlab online, (and if this is supported by [go-gitlab](https://github.com/xanzy/go-gitlab)) users should have the possibility of choosing whether they create a note as a non-resolvable comment or a thread. This could be implemented as another field in the note popup next to "Draft", something like "Type" with the values "Comment" and "Thread".

I've also just noticed, that Gitlab also allows creating "Internal notes". `gitlab.nvim` should probably at least show the `internal` flag on the note, ideally, also enable users to create internal notes (that could be another field in the note popup).