Add initial support for Bootstrap 5

[greyli]

After this PR merged, we can start to work on the macros one by one to adapt them to support Bootstrap 5.

It brings clear names for different Bootstrap versions. You can use Bootstrap5 for Bootstrap 5:

from flask_bootstrap import Bootstrap5

bootstrap = Bootstrap5(app)

Import templates from the bootstrap5/ path:

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

And if you want to use Bootstrap 4, just use Boostrap4 and boostrap4/:

from flask_bootstrap import Bootstrap4

bootstrap = Bootstrap4(app)

{% from 'bootstrap4/form.html' import render_form %}

The old names (Bootstrap and bootstrap/) will throw out a warning from version 2.0.

Todo:

• Refactor the base class to support multiple major Bootstrap versions.
• Refactor tests to support test multiple Bootstrap versions.
• Add tests for Bootstrap5 load_* methods.
• Move Bootstrap 4 resources to subfolder bootstrap4.
• Add Bootstrap 5 resources.
• Move Bootstrap 4 templates to the boostrap4 folder.
• Add warning for Boostrap class and template path bootstrap/.
• Add Bootswatch themes files for Bootstrap 5.
• Add example application for Bootstrap 5.
• Update docs and README.
• Add docs for Bootstrap 5 support.