Add Optional Roving Tabindex to the Toolbar

[colfin-96]

Hi there,

I use quill a lot and recently forked off the project to add some logic for a roving tabindex in the toolbar. The idea is to make the UX a bit better for users so that they don't have to tab through each item of the toolbar in order to reach the editor.

So currently, if you have 5 items in the toolbar (bold, italic etc.), you would have to tab 6 times to reach the editor. With a roving toolbar, you only have to tab twice. To navigate inside the toolbar, you use the arrow keys (it's actually exactly how it works here in GitHub too).

I have actually already implemented it (#4557) but then I saw that a discussion should take place first in order to see whether this feature is aligned with the goals of quill (https://github.com/slab/quill/blob/main/.github/CONTRIBUTING.md). The PR is still in draft mode because of this and because I don't have tests etc. yet, but I'm working on that.

A quick summary of what I implemented in the PR so far:

• optional roving tabindex, which can be enabled by adding a CSS Class '.roving-tabindex' to the parent div

Open Todos:

• adding tests
• add demo and/or documentation to website? Not sure if this is needed but might be quite helpful as I'm sure other users would like this behaviour too
• check the linter / coding styles