Could component mode be detected and shown?

[brunnerh]

Description

During the transition to Svelte 5, if the mode is not globally enforced, unintentionally being in one mode or the other could be undesirable.
E.g. legacy mode leads to coarse grained reactivity which can cause issues and just has worse performance.

Seen unintended mode switches happen when people refactored the last variable using a rune into a separate file.

Proposed solution

If mode is detectable, show mode via CodeLens at the top of the file.

Ideally this would be configurable, e.g. via an array specifying the modes one is interested in.
Something like:

{
  "svelte.showComponentMode": ["legacy", "runes"],
}

Alternatives

There might be other ways of making users aware of the mode or mode switches happening.
E.g. on switch a notification could be shown, but this seems too intrusive.

Additional Information, eg. Screenshots

No response