fix: cache nested loops

[pauldambra]

we're clearly hitting a pathological case in the CSS processing for some folks
my guess is something is repeating the check due to run away mutations or something in a tight loop... or maybe it is just bad at very large strings 🤷

quoting my old colleague Chris Sewart about peformance

you can do it faster
do it less
or not do it

let's cache our results as we go so that we only run the nested check or the regex replace once per input

you can see the test here PostHog/posthog-rrweb#17

we see results like

First execution time: 3.3014169996604323ms
Average execution time for the second execution: 1.8079558699950575ms

that second execution will be O(1)
while first execution is O(n)

so where O(n) is large this should be a much bigger improvement

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Jan 22, 2025 7:43pm

[github-actions]

Size Change: +781 B (+0.02%)

Total Size: 3.28 MB

Filename	Size	Change
dist/all-external-dependencies.js	209 kB	+111 B (+0.05%)
dist/array.full.js	370 kB	+111 B (+0.03%)
dist/array.full.no-external.js	369 kB	+111 B (+0.03%)
dist/module.full.js	370 kB	+111 B (+0.03%)
dist/module.full.no-external.js	369 kB	+111 B (+0.03%)
dist/recorder-v2.js	116 kB	+113 B (+0.1%)
dist/recorder.js	116 kB	+113 B (+0.1%)

ℹ️ View Unchanged

Filename	Size
dist/array.full.es5.js	266 kB
dist/array.js	182 kB
dist/array.no-external.js	180 kB
dist/customizations.full.js	13.8 kB
dist/dead-clicks-autocapture.js	14.4 kB
dist/exception-autocapture.js	9.48 kB
dist/external-scripts-loader.js	2.64 kB
dist/main.js	182 kB
dist/module.js	182 kB
dist/module.no-external.js	180 kB
dist/surveys-preview.js	67.5 kB
dist/surveys.js	64.2 kB
dist/tracing-headers.js	1.76 kB
dist/web-vitals.js	10.4 kB

_{compressed-size-action}

[veryayskiy]

//

[rafaeelaudibert]

Memory usage will increase, but that should be fine, lgtm :)