Use yarn for cross-workspace script running

[kumavis]

Migrates cross-workspace script running to yarn workspaces.

lerna is still used for:

• ci prepack check after packing
• publishing
• cleaning (removing all node_modules)

etc:

• added test:xs script in root
• deprecated "yarn run update" in root

[mhofman]

This might break agoric-sdk integrations with Endo. We'll need to run some integration on the agoric-sdk side before this can merge

[kumavis]

@mhofman unsure how this would affect agoric-sdk. lerna is not removed and the change is primarily how the scripts are invoked. where can i learn more?

[kumavis]

unfortunately, yarn did not solve the main issue i wanted it to, indicating which workspaces failed
yarnpkg/berry#6535

[kumavis]

I encountered a "cover" CI issue twice in a row (once on first run, then on retry failed). Retrying all fixed it, so I suspect something to do with the cache (?)
unsure what else it could be. race condition?

[turadg]

I don't see a way this could break agoric-sdk's Endo integration but here's a test to be sure: Agoric/agoric-sdk#9385

[turadg]

Thanks for reducing reliance on Lerna.

Code looks good, CI green here and in agoric-sdk integration branch

[kumavis]

(rebased)