disable migrate button if component is already migrated

[Rich-Harris]

from a UI perspective the most sensible thing to do would be to disable it when the runes lightning icon is orange, though it would be more correct (and more finicky) to proactively migrate everything in the background, and disable it if the migrated code is identical to the input code

[Rich-Harris]

also it should probably indicate if the migration succeeded — sometimes it does nothing because nothing needed to happen, and it's kinda confusing

[Rich-Harris]

as of #430, the migrate button is disabled if no migration is possible, or the migration has no effect