chore: build an es5 bundle and move main to es6

[pauldambra]

stacked on top of #1479

not supporting IE11 is a 10% bundle saving
so, we should move anyone that needs to support IE11 to a new bundle

[vercel]

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Oct 17, 2024 0:03am

[github-actions]

Size Change: -283 kB (-9.14%) ✅

Total Size: 2.81 MB

Filename	Size	Change
dist/all-external-dependencies.js	172 kB	-20 kB (-10.45%)	👏
dist/array.full.js	324 kB	-32.6 kB (-9.13%)	✅
dist/array.full.no-external.js	323 kB	-32.5 kB (-9.13%)	✅
dist/array.js	155 kB	-16 kB (-9.34%)	✅
dist/array.no-external.js	154 kB	-15.9 kB (-9.35%)	✅
dist/exception-autocapture.js	8.75 kB	-2.6 kB (-22.88%)	🎉
dist/external-scripts-loader.js	2.17 kB	-180 B (-7.66%)	✅
dist/main.js	156 kB	-16 kB (-9.31%)	✅
dist/module.full.js	324 kB	-32.6 kB (-9.13%)	✅
dist/module.full.no-external.js	323 kB	-32.5 kB (-9.13%)	✅
dist/module.js	155 kB	-16 kB (-9.35%)	✅
dist/module.no-external.js	154 kB	-15.9 kB (-9.36%)	✅
dist/recorder-v2.js	92.6 kB	-18.1 kB (-16.36%)	👏
dist/recorder.js	92.7 kB	-18.1 kB (-16.35%)	👏
dist/surveys-preview.js	56.6 kB	-3.19 kB (-5.33%)	✅
dist/surveys.js	62.1 kB	-3.9 kB (-5.92%)	✅
dist/tracing-headers.js	1.33 kB	-7.03 kB (-84.1%)	🏆
dist/web-vitals.js	10.3 kB	+8 B (+0.08%)

ℹ️ View Unchanged

Filename	Size
dist/array.full.es5.js	248 kB

_{compressed-size-action}

[daibhin]

The size savings here are incredible!