To support easier migration of deprecating old zsh options. We can
adjust the internal priorities to fit within the `mkBefore` (500) and
default (1000). Makes it easier for `initExtraFirst` migrationg to
`mkBefore` and `initExtra` to default prio.
* zsh: add initContent option for custom .zshrc content insertion
- Users can add content anywhere by using `lib.mkOrder`, `lib.mkBefore`
and `lib.mkAfter` custom configurations.
- Add test cases to verify the insertion of content before and after
existing configurations in `.zshrc`.
consolidate zshrc content tests into a single priorities test
