chore: Avoid inexact object types

[nmn]

A follow up from my previous PR #81

This PR removes a few cases where inexact object types are used and replaces them with
{ +[string]: mixed }

This is a quirk of how inexact object types work. They do not allow reading any of those additional keys which may or may not exist on the object. They only loosen the constraints for what objects can be given when such a type is used.

This is particularly problematic when {...} is used as a stand-in for "any object". According to flow, an exact object type does not satisfy {...}.

Using { +[string]: mixed } fixes all of these problems and works in all scenarios for at least the last 30 versions of Flow.

[github-actions]

compressed-size: runtime library

Size change: 0.00 kB
Total size: 18.80 kB

View unchanged

Filename: gzip (minify)	kB size	kB change	% change
./packages/react-strict-dom/dist/dom/index.js	2.99 (9.00)	0.00 (0.00)	0.0% (0.0%)
./packages/react-strict-dom/dist/dom/runtime.js	0.95 (2.33)	0.00 (0.00)	0.0% (0.0%)
./packages/react-strict-dom/dist/native/index.js	14.85 (46.47)	0.00 (0.00)	0.0% (0.0%)