Adopt Rapid's issue level 'suggestion' for lower severity issues

[tordans]

Context

Rapid recently added a third issue type "Suggestion" to the existing Warning and Errors.

• This is what it looks like Validator: Add lower issue severity for 'suggestions' facebook/Rapid#1623
• Most of the code changes are facebook/Rapid@d758851 and facebook/Rapid@3e44d58

We have an existing issue on this in #6101 which links to quite a few other issues that would benefit from this feature.

The Rapid code changed quite a bit since it's fork but in general the existing implementation should make it a lot easier to implement this in iD.

Goal

• Test out Rapid's implementation and discuss if this is what we want for iD.
  This is about the issue panel itself but also about the sidebar, status bar, changelog tags and other parts that the issues system touches.

• Adopt the changes from Rapid to iD.
  This might require to change the implementation from what Rapid does in case this used Rapid specific code path that iD does not have.

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FYI I added this as a suggestion for a GSoC Project (as a package with #10757): https://wiki.openstreetmap.org/wiki/Google_Summer_of_Code/2025/Project_ideas#iD_editor