Finalize HMR support

[fregante]

Context:

• https://github.com/pixiebrix/pixiebrix-extension/pull/7598

The feature was added as an option because it's not stable:

• [x] Page Editor: supported
• [x] Options: supported
• [x] Sidebar: supported
• [ ] Content script: on most sites it causes reload loops

The last point is severe enough that HMR cannot be the default until that one is fixed, but HMR is really useful when working on the UI of the other 3 contexts.

I'm not prioritizing this at the moment, but since the app uses an old HMR method, we should probably fix it eventually, using the same method in both. See:

• https://github.com/pixiebrix/pixiebrix-extension/pull/7763
• https://github.com/pixiebrix/pixiebrix-app/pull/4857
• https://github.com/pixiebrix/pixiebrix-app/issues/5044