Make enableBigIntSupport www dynamic

[eps1lon]

Feature: #24580

[react-sizebot]

Comparing: 9ffe910...e46b1e8

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	=	176.83 kB	176.83 kB	=	54.91 kB	54.91 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	173.55 kB	173.55 kB	=	54.11 kB	54.11 kB
facebook-www/ReactDOM-prod.classic.js	+0.11%	594.04 kB	594.67 kB	+0.08%	104.37 kB	104.45 kB
facebook-www/ReactDOM-prod.modern.js	+0.11%	577.30 kB	577.93 kB	+0.06%	101.42 kB	101.48 kB
test_utils/ReactAllWarnings.js	Deleted	66.87 kB	0.00 kB	Deleted	16.36 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
facebook-www/ReactServer-prod.modern.js	+0.77%	16.54 kB	16.66 kB	+0.69%	4.34 kB	4.37 kB
facebook-www/React-prod.modern.js	+0.63%	20.29 kB	20.42 kB	+0.59%	5.09 kB	5.12 kB
facebook-www/React-prod.classic.js	+0.62%	20.58 kB	20.71 kB	+0.60%	5.15 kB	5.18 kB
facebook-www/React-profiling.modern.js	+0.61%	20.73 kB	20.85 kB	+0.62%	5.16 kB	5.20 kB
facebook-www/React-profiling.classic.js	+0.60%	21.01 kB	21.14 kB	+0.65%	5.23 kB	5.27 kB
test_utils/ReactAllWarnings.js	Deleted	66.87 kB	0.00 kB	Deleted	16.36 kB	0.00 kB

Generated by 🚫 dangerJS against e46b1e8

[kassens]

Sorry messed up the rebase merge resolution.

[eps1lon]

@kassens It can now be landed cleanly. When you land it, would you take care of rolling it out at Meta?

[jackpope]

This change landed in #28617