Update bundling and vendoring (JS)

[trallard]

This PR is an attempt to improve our approach to bundling theme assets. As you go through the PR, you will notice some changes, such as:

• Removal of duplicate imports (per https://github.com/pydata/pydata-sphinx-theme/issues/1788)
• Instead of copying Font Awesome files around, these are now vendored similarly to how we do with Bootstrap
• There are also some optimisations which should help with performance (making the theme a bit snappier) and better bundling

Closes: https://github.com/pydata/pydata-sphinx-theme/issues/1613 Closes: https://github.com/pydata/pydata-sphinx-theme/issues/1788