Add `MultipleFileField` and support validating multiple files

[greyli]

Continue #338

• Add the MultipleFileField field.
• Update FileRequired, FileAllowed, and FileSize to make them work with multiple files.
• Add tests for using Length with the files field.

fixes #337 fixes #393

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [x] Add or update relevant docs, in the docs folder and in code.
• [x] Add an entry in docs/changes.rst summarizing the change and linking to the issue. Add .. versionchanged:: entries in any relevant code docs.

[greyli]

The failed style check is handled in #557.

[greyli]

Not sure why the warnings from flask-babel are treated as errors: https://github.com/wtforms/flask-wtf/actions/runs/5623535426/job/15238419433?pr=556#step:6:129

[azmeuk]

This can be useful to force us to fix the deprecation warnings emitted by other libraries.

However for this one there is not much that can be done on our side since the warning is emitted because of flask-babel: https://github.com/python-babel/flask-babel/pull/230 cc @tktech is the PR there ok to you?

Anyways, I checked your branch (with the flask-babel patch) and this looks all good to me :ok_hand:

[azmeuk]

@greyli you are maintainer, do you want to merge this PR or do you need other pairs of eyes to review it?

[greyli]

Yes, I do want to merge this. Just went through all the changes again and fixed a typo in the docs.

[azmeuk]

@greyli Do you think it this is ready to merge now? Do you want me to merge it and cut a release?

[greyli]

@greyli Do you think it this is ready to merge now? Do you want me to merge it and cut a release?

I think so. Thanks!