feat(rxjs) update rxjs to be a peerDependency (v6 or v7)

[krisdages]

Allows consuming projects to upgrade to rxjs 7 without a version resolution override for aurelia-store. Without this change, build systems would bundle both versions and warn of the duplication. Modifications:

• Install RXJS 7 as a devDependency for the local build
• Update TypeScript to 4.3 for compatibility with RXJS 7
• Update other build devDependencies to fix build errors with new versions

Possible breaking changes: rxjs would have to be installed in the consuming project, which might not be automatic with older versions of npm. Anyone using the plugin would likely already be using RXJS to interact with the store, though.

[krisdages]

I signed the CLA a few years ago for an update to the logging library. Hopefully it's still valid, but I can do it again if I need to.

Is the CI error below something I can resolve? The tests themselves passed, and the uncovered lines were already there in master.

[zewa666]

Thanks for the contrib @krisdages. The CLA is no longer needed so all good. Updating TS will certainly result in a breaking change so I'll give your PR a proper look next week