chore: migrate to expecttype eslint rules

[43081j]

This migrates to using an eslint plugin for $ExpectType and friends so we can do it as part of the lint script.

For now, typescript had to be added to the root project since the rxjs site is still on an older typescript which confuses eslint (since it ends up being hoisted by yarn).

In future, we should probably keep the two projects in sync, using the same typescript version. However, this will mean bumping the angular version in the rx site (which is its own chunky job).

Questions

• do we still need tsconfig.types.json? and tsconfig.types.spec.json. im not sure what use these have anymore, as we use a separate mocha tsconfig during testing anyway

[43081j]

@benlesh fyi this seems to have also caught some leftover dtslint assertions for things that have since been removed

e.g. every supporting a thisArg

the jsdoc was also mentioning those params still, so i removed that in this PR.

pinging you because you made those changes iirc, so want to be sure this is right

[43081j]

any chance you can take a look at this @benlesh ? 👀

low priority ofc, assuming you do want the change!