New Core Admin Menu Functions -- `add_menu_link` and `add_submenu_link`

[georgestephanis]

Currently if a plugin would like to add admin menu links that go to a url, instead of a registered admin page, there are only two options:

Option 1: Register a dummy admin page, and add an action that sets up a wp_redirect on load.
Option 2: Mess with the menu globals directly.

Neither of these is a particularly good option, and encouraging messing with globals directly feels bad.

To that end, this will let folks add admin menu links -- whether to plugin support forums, to the customizer even (there is no good way to add deep links to the customizer currently), and the like.

Is this something we'd like to support?

[swissspidy]

Related: #12718