Fixes #10404

Co-authored-by: Marc Cornellà <hello@mcornella.com>

Apple changed the name of their operating system from OS X to macOS a number of years ago. This was overdue!

As per issue  #10311

* refactor(osx): rename `osx` plugin to `macos`
* refactor(macos): Add symbolic link from old `osx` plugin name.

Closes #8370

Fixes #10350

In recent zsh versions, `${(@ps:$sep:)var}` where $sep is a variable containing
a separator string and $var is a string with multiple values separated by $sep,
the `p` flag makes zsh correctly expand $sep before splitting $var. In versions
older than 5.0.8, this doesn't happen, so we use `eval` to get the same effect.

Fixes #10345

Fixes #8492

Co-authored-by: Marc Cornellà <hello@mcornella.com>

BREAKING CHANGE: all `gem` aliases that started with `g` now start
with `ge` to fix conflicting names with the `git` plugin. Also, the
`ghlp` alias is now renamed `geh`. Have a look at the plugin README
for more information.

Fixes #10320

Mostly empty commit to fix the changelog for merge at 6f4c7f64

Co-authored-by: Marc Cornellà <hello@mcornella.com>