Improve TypeScript for v4.x

[eXamadeus]

See #480, #484 for more context and initial PR

Upgrade Types without breaking changes

We'd like to perform any updates/fixes to TypeScript that won't result in breaking changes.

Goals

• fix bugs w/ typings ([TypeScript] defaultMemoize errors #384 is an example)
• support TypeScript 3.9+ and validate it in CI
• remove failing tests for 3.1 changes (As of TypeScript 3.1.1, types erroneously fall back to arity-agnostic createSelector signature when used with arrays #420) OR fix those issues in the typings

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Some caveats

• ci: replace TravisCI with GitHub Actions #483 replaces the CI pipeline with GitHub Actions. We need to support testing multiple versions of TypeScript with that new CI structure (this is already worked out in refactor(typescript): upgrade to TS 4.1, simplify and generalize types #480)

[markerikson]

I'd like to pick up with this effort and push things forward. Where do things stand atm?

[markerikson]

Merged #486 , so I'm calling this good.