feat(vModel): handle true-value and false-value

[cexbrayat]

I noticed that the current v-model does not handle true-value and false-value for checkboxes as we had in Vue 2.x.

This is a rough attempt to implement it, just to start a discussion.
Is this a feature you plan to keep in v3? If so, does this look the right way to handle it?

[posva]

Aren't we missing passing objects to true-value and false-value? If we get through HTML attributes, we can only have strings, no numbers, no objects

[yyx990803]

Good catch! As @posva noted, we also need to handle e.g. :true-value="someObject" so we need to force set true-value and false-value as DOM properties on <input> elements - which means we will need a special case for that here.

Then in vModel.ts we will need a helper like this:

function getCheckboxValue(el, checked) {
  const key = checked ? 'true-value' : 'false-value'
  return key in el ? el[key] : checked
}

[cexbrayat]

@posva @yyx990803 Thanks for the feedback!

I added a unit test and implemented a special case in patchProp, let me know if this how you intended it.

[vue-bot]

Hey @cexbrayat, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚