Wait until all core extensions are loaded before loading user extensions.
This makes menu ordering more consistent, and fixes bug #5442 where the
order of registration matters.

Also change extension loading to work with arrays most of the time, avoiding
bugs if there are ","s in pathnames.

And fix stale comment in ExtensionManager.
Ensure linting results update when a new linter is registered that might
affect the current file. Linters can register very late due to on-the-fly
Extension Manager installs.
Merge pull request #5493 from adobe/pflynn/load-core-exts-first

Load core extensions before user extensions