Add 'rush-pnpm up' support for catalogs

[benkeen]

Addresses: #5578

Right now, when you update packages in a Rush/pnpm repo using rush-pnpm up, it updates the catalog entries in common/temp/pnpm-workspace.yaml, but doesn't update Rush's actual catalog, stored in pnpm-config.json, so the update command doesn't fully work. This has been supported since pnpm@10.12.0. This PR enhances it to update the pnpm-config file too.

I also updated it to fix a small issue where JsonFile.save could throw an error due to an undefined $schema property.

Besides the tests, I've ran manual checks with our own Rush repo to confirm the catalog is getting updated properly.

[benkeen]

Howdy @iclanton, @dmichon-msft - don't suppose either of you have context or time to look at this?

[iclanton]

Shouldn't users be running rush update instead of rush-pnpm update?

[benkeen]

Huh, neat. I never associated those two commands in my mind.

rush update has a very limited, custom interface for Rush itself. What I'm trying to do is offer up all the extra options with pnpm update, for updating subsets of dependencies, like:

pnpm update -r "@babel/*"

That could always be done with rush-pnpm (but not rush update); this PR just lets that command handle catalogs too.

Tell me if I'm being barmy.