Recast Cask::DSL::Installed as Cask::Staged

Change namespace for clarity, as we already have Cask::Installer,
and are about to add Cask::DSL::Installer.

Move to parent because these methods are not DSL-specific.
use path_environment_variable for caveats test

instead of manual_installer
recast install_script to installer :script

* document installer :script.  install_script was never documented
doc: remove needless column from table
dsl: add `installer :manual`

a toplevel artifact, intended to replace `manual_installer`,
which is currently implemented within the `caveats` mini-DSL
Merge pull request #6660 from rolandwalker/installer_stanza

DSL: add `installer` stanza