easier to setup/use node/npm as actions/setup-node@v2 already handles
the npm auth step using the NODE_AUTH_TOKEN env var. Also, the wombat
dressing room proxy can be problematic when using yarn.

npm install:
--no-verify-access: verification through the wombat proxy was not happy
--no-save, --no-package-lock: lerna does not like untracked files
--legacy-peer-deps: npm7 cli bombs out otherwise (react library gremlins)

package.json:

- adding a top level package to release-please so we can key off a single
  tag for publishing the typescript packages (e.g. sdk-codegen-all-v1.1.0)
- removed unneeded `yarn` command from build scripts

prepublishOnly support: setup the testenv

lerna.json: there are still cases where we'll do a
```
lerna publish --canary
```

(potentially with the `--no-git-tag-version` flag?) so we want each
package version to just bump independently in that case.