`deprecated.txt` is a place for deprecated tags to live.
- Encourages aggressive documentation of deprecations without cluttering
the main help files.
- Provides a single browsable reference of all deprecations.
Other changes:
- Move tags to doc/vim_diff.txt.
- Remove doc/quotes.txt. It has little historical value, except maybe the
Larry Wall quote.
- Use execute() instead of redir
- Fixed logic on suboptimal pyenv/virtualenv checks.
- Move system calls from strings to lists. Fixes#5218
- Add highlighting
- Automatically discover health checkers
- Add tests
Helped-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Helped-by: Tommy Allen <tommy@esdf.io>
Closes#4932
