Add pending state to useFormState

[rickhanlonii]

Overview

Adds a pending state to useFormState, which will be replaced by useActionState in the next diff. We will keep useFormState around for backwards compatibility, but functionally it will work the same as useActionState, which has an isPending state returned.

[react-sizebot]

Comparing: 0066e0b...97a9ef2

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	+0.05%	177.10 kB	177.19 kB	+0.03%	55.19 kB	55.21 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	+0.05%	177.64 kB	177.73 kB	+0.03%	55.52 kB	55.54 kB
facebook-www/ReactDOM-prod.classic.js	+0.12%	594.36 kB	595.05 kB	+0.09%	104.96 kB	105.05 kB
facebook-www/ReactDOM-prod.modern.js	+0.12%	577.62 kB	578.31 kB	+0.09%	102.01 kB	102.10 kB
test_utils/ReactAllWarnings.js	Deleted	66.60 kB	0.00 kB	Deleted	16.28 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js	+0.27%	29.76 kB	29.84 kB	+0.35%	7.66 kB	7.69 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js	+0.27%	29.76 kB	29.84 kB	+0.35%	7.66 kB	7.69 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js	+0.27%	29.76 kB	29.84 kB	+0.35%	7.66 kB	7.69 kB
facebook-react-native/react-test-renderer/cjs/ReactTestRenderer-prod.js	+0.21%	315.59 kB	316.25 kB	+0.18%	55.92 kB	56.02 kB
facebook-react-native/react-test-renderer/cjs/ReactTestRenderer-profiling.js	+0.20%	331.49 kB	332.15 kB	+0.17%	58.36 kB	58.46 kB
test_utils/ReactAllWarnings.js	Deleted	66.60 kB	0.00 kB	Deleted	16.28 kB	0.00 kB

Generated by 🚫 dangerJS against 97a9ef2

[acdlite]

What a pleasure to review!

[rickhanlonii]

I refuse to believe I did this correctly, but I'm merging anyway