breaking: apply fallback value every time in runes mode

[dummdidumm]

closes #9948

Draft mode because I came across an interesting question with regards to accessors and default values. Right now, setting a component property to undefined through its accessor (component.property = undefined) will set it to actually undefined instead of the default value. One could argue this is correct since implementation-wise this isn't looking for what's coming in from the property, instead it's updating the internal state with a new value. Semantically one could argue it should apply the fallback value (change should be straightforward, change the setter from set x($$value) {..} to set x($$value = fallback) {..}) since it's setting the property, just through another path. Semantically one could also argue that setting it through the accessor is more direct than through a property, and so it shouldn't apply.
We need to make a decision here. I slightly lean towards "applies default value".
Update: decided

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[Rich-Harris]

I slightly lean towards "applies default value"

same. I also lean strongly towards 'nuke accessors from orbit as soon as we can'

[Conduitry]

Yup, I agree with all of the above - apply the default value, and nuke accessors ASAP. I think it's acceptable to have a getter return a different value from the one you just called the setter with.