Added model field name actions

[Aquafina-water-bottle]

Use case

I've been working on a script to update a model in place, and I occasionally modify the field names (positioning, etc.) of the model. It would be convenient to use AnkiConnect to do just that.

Additional notes

• I'm currently working on fleshing out the documentation, hence why this is a draft request. edit: documentation added.
• Seeing that the multi action exists, I figured that each action only affecting one field is sufficient.
• I'm personally conflicted on the exact naming of the actions. Perhaps something along the lines of renameFieldName or renameModelFieldName would be better. Do you have any ideas?
• I was wondering if certain errors should be silent. Currently, the modelFieldAdd action does not show an error if the field already exists, and continues to reposition the field. However, the modelFieldRemove (and the other 2 actions) will show an error if the field does not already exist. Is this behavior good?

Thanks!

[FooSoft]

Hey @Aquafina-water-bottle I think this PR looks good. The API surface (for better or worse) has grown organically, and some things are more consistent than others. I think naming and error handling is fine the way you have it now. Thanks for the contribution!