Add Bootstrap 5 support

[realJustinLee]

No description provided.

[TitaniumHocker]

Also don't forget about that:
thomaspark/bootswatch#1056

[greyli]

I'll work on this issue next month. Here are some initial ideas:

• I will try to make the Bootstrap 5 support happened in the same project.
• Provide a Bootstrap5 extension class, it has different resources methods for Bootstrap 5.

from flask_bootstrap import Bootstrap5

bootstrap = Bootstrap5()

• Add new versions of macros in a new bootstrap5 folder:

{% from 'bootstrap5/forms.html' import render_form %}

[greyli]

The initial support for Bootstrap 5 was added (#161), the macros upgrade progress is in #162.