chore: backwards compat for dash-case warnings

[dummdidumm]

Compiler warnings that are dashed are also taken into account for ignoring warnings for backwards compatiblity (else many apps would be littered with a11y warnings now).
Also adds logic to the migration script to convert these to underscores.
Along the way I discovered that attribute a11y warnings were unsilenceable, the path upwards traversal was more robust in that regard.

Related to #11414
fixes #11482

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🦋 Changeset detected

Latest commit: 546cc7d

The changes in this PR will be included in the next version bump.

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[Rich-Harris]

I think we should probably try and remap the warnings that have changed beyond just _ -> -, especially for the purposes of the migration script — I don't think there's that many. I've been meaning to get round to this (should probably have taken notes while I was doing the overhaul, but oh well...)

I also think we should only do this in non-runes mode. In runes mode we should warn on any unknown codes.

[dummdidumm]

• only ignoring the old warning codes in legacy mode now
• adjusted the ignore logic while I was in that area (using the same global context approach that is already used for warnings)
• started a map of new codes -> old codes (and vice versa). Feel free to add more codes you know you have renamed - but don't let perfect be the enemy of the good, I think it's more useful to merge this now and add those when we come across them. People have their console littered with warnings right now and it would be good to get this out quickly

[Rich-Harris]

I think it would be simpler and more helpful to warn on <!-- svelte-ignore a11y-missing-attribute --> in runes mode, and leave the other warning untouched

[Rich-Harris]

(working on this)

[dummdidumm]

Closing in favor of the other PRs that did this.