Enable Flow lint rule implicit-inexact-object and fix issues

[nifgraup]

Allows dependant code to enable option exact_by_default.

https://medium.com/flow-type/on-the-roadmap-exact-objects-by-default-16b72933c5cf
https://medium.com/flow-type/how-to-upgrade-to-exact-by-default-object-type-syntax-7aa44b4d08ab

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[JedWatson]

Hey @nifgraup sorry but we've ended up with a lot of conflicts here, before the PR came up for review.

At first glance, I'd be happy to merge this (and thanks for picking it up!) but would you be up for resolving the conflicts?

Thanks 🙏

[Methuselah96]

@nifgraup It's worth noting that we are planning on transitioning away from Flow and to TypeScript in 5.0 (#4142). It's fine if you want to update this PR, just be warned that the changes will likely be overriden by TypeScript in the (relatively) near future.

[Methuselah96]

Thanks for the work on this, we've recently converted the code over to TypeScript (#4489) in preparation for v5 so the Flow types will no longer be relevant.

[nifgraup]

Sounds like a good conclusion. New Flow libdefs can be derived from the TS code.