build: compile/watch sass with new npm scripts

`paver` commands are deprecated for managing static assets. Starting in
Sumac, only `npm run` commands will be supported for managing static
assets.

To ease the transition, both `paver` and `npm run` commands will work in
Redwood. However, we want to stop using the *implementations* of the
`paver` asset commands right now, as they are blocking the Python 3.11
upgrade. This will also make the removal of `paver` commands more
straightforward come Sumac.

So, this commit turns these commands/functions:
* paver compile_sass (used by configuration)
* paver watch_sass (used by configuration and devstack)
* pavelib/assets.py:_compile_sass (used by Tutor)

into very thin wrappers around the new `npm run` commands. Each of these
paver routines now raise a loud deprecation warning, including a message
of the `npm run` command that the operator can switch to.
We expect no impact to site operators or end users.

openedx#31895