JS 340KB -> 210KB by dropping legacy polyfills

JS 340KB -> 210KB by dropping legacy polyfills
The build transpiled to ES5 and shipped 130KB of polyfills to 100% of users — but analytics showed 99.6% on evergreen browsers.
— Switched to module/nomodule: modern bundle to evergreen, legacy only to the 0.4%.
— Modern bundle: 340KB | 210KB (gzip 96KB | 58KB).
Parse/compile on median mobile: 280ms | 160ms
p75 INP: 210ms | 150ms
Takeaway: 130KB of polyfills served to 99.6% of users who never needed a byte of them.