docs: remove mention that some browsers do not support esm in service workers

[teemingc]

closes #15069

ESM in service workers is now baseline https://caniuse.com/wf-js-modules-service-workers

Also restructures the service worker docs to deduplicate introducing the manual registration concept

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[svelte-docs-bot]

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[benmccann]

@Rich-Harris could you test that everything works as expected in Firefox as our resident Firefox expert?