The top-level template object `extra_javascript` gets soft-deprecated because it still reports only a list of plain strings.
Themes need to take action and pick up the new config keys from `config.extra_javascript` instead.
These files will not be picked up into the `Files` collection and as such, also the final built site.
Also adds the ability to *un*-ignore the files that MkDocs forcibly ignores.
The user story is that the following command should let you "just build" any MkDocs site:
pip install $(mkdocs get-deps) && mkdocs build
This cross-references 2 files:
* mkdocs.yml - `theme`, `plugins`, `markdown_extensions`
* projects.yaml - a registry of all popular MkDocs-related projects and which entry points they provide - downloaded on the fly
-and prints the names of Python packages from PyPI that need to be installed to build the current MkDocs project
