switch to rush

[mmkal]

tracking issue for the switch from lerna/yarn to rush

Motivation:

• learn about rush
• enforce more rigour/stop getting away with leaky abstractions thanks to yarn hoisting
• take advantage of rush's tooling:
  • more readable log output
  • enforce consistent versions
  • parallelise builds intelligently
  • use pnpm's better node_modules symlinking system
  • automate publishing flow
  • (eventually/maybe) deploy one or more projects (small documentation website? serverless function fetching code coverage results, without using codecov.io?)
  • future features - seems to be more actively maintained, and more ambitious than lerna

tasks:

• initial rush setup
• enable ci - build/lint/test using rush
• enable code coverage
• dev tooling - get eslint working locally
• documentation - at least a link and "use rush"
• enable strictPeerDependencies
• enable ensureConsistentVersions
• re-enable renovate
• use locally-built lint rule (eslint-plugin-codegen). turned off for now
• automatically push change files based on PRs
• automatically publish packages from a manually-triggered workflow
• automatically create github releases
• document manually-triggered publish workflow
• re-enable pre- and post-publish scripts
• add clean to build script (maybe by moving to heft)
• cache build outputs so CI can build only what's changed in most PRs
• cache dependencies to make installs faster
• add tests for rig init script, and enable coverage
• add codegen for rush.json - glob for packages like lerna does and add them to projects