feat: default values for form elements

[dummdidumm]

WIP

closes #8220
closes #9230

This implements default values for form elements. These default values take effect when the reset method of the surrounding form is invoked.

• for input with value/checked it's defaultValue/defaultChecked
• for input with bind:group it's checked (hydration not working yet, not working for dynamic yet)
• for select it's selected on the option elements (hydration not working yet, not working for dynamic yet)
• defaultValue/defaultChecked on the server -> use if value undefined/falsy?

Open questions:

• for defaultValue/defaultChecked the value of that takes precedence on the initial render in case the value/checked value is undefined/null. That also means that in case of bind:value/checked the default value will be propagated back to that variable. Ok?
• In case that's ok, should we do the same for bind:group? It's tricky there because for a group of checkboxes, we currently require an array to be set (else you get runtime errors), and so the value isn't falsy, and the empty array could be on purpose / hard to say if the default checked should take precedence there initially
• Same question for bind:value on selects with multiple, with same problem as for bind:group on checkboxes

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