Update runtime.onLoadedReason to match runtime.OnInstalledReason

[carlosjeurissen]

This changes the reasons to the present simple as agreed during the 2025-12-18 meeting to align with runtime.onInstalledReason

This PR also adds the browser_update and shared_module_update reasons.

Alternatively we could also add these reasons to runtime.onInstalledReason and use it for onExtensionLoaded as well.

@oliverdunk when this gets implemented in Chrome, is Google in favour of using browser_update instead of chrome_update as reason at least in the onExtensionLoaded event?

@oliverdunk once merged we should update https://issues.chromium.org/issues/41486131

Lastly, there is open discussion on what should happen if multiple reasons apply. See:
#353 (comment)

[oliverdunk]

As far as I know, shared modules are a Chrome-only concept (and aren't currently supported for extensions in the Chrome Web Store). @Rob--W / @xeenon: Is there any interest in mentioning those here or should we just make that an additional Chrome-specific reason?

[oliverdunk]

@oliverdunk when this gets implemented in Chrome, is Google in favour of using browser_update instead of chrome_update as reason at least in the onExtensionLoaded event?

I'll reach out to Devlin about this and follow-up. I assume we will be supportive though :)

[oliverdunk]

I'll reach out to Devlin about this and follow-up. I assume we will be supportive though :)

Confirming that we are ok using browser_update for new events. We don't plan to change any existing ones in the short term.