Add popover attributes

[knpwrs]

This PR adds popover attributes as detailed here: https://developer.mozilla.org/en-US/docs/Web/API/Popover_API/Using

[markuswustenberg]

Hi @knpwrs, thanks for this.

This is the second attribute in a short time that allows zero or one argument(s) (popover is the same as popover="auto"). I'm wondering whether we should start adding a third form of attribute in the html package, with varargs values, similar to how gomponents.Attr itself works. And then panic if there's more than one value. Thoughts?

[knpwrs]

I think that makes sense. Should this attribute and the others be moved somewhere for locality or should I just update this in place?

[markuswustenberg]

@knpwrs I think in-place is fine. But it needs to go into a new test which:

1. Tests that it works like a boolean attribute
2. Tests that it works as a key-value attribute
3. Tests that it panics with more than one value given

Might as well start a new table-driven test, I think more attributes like these will show up.

Are you up for that? Otherwise let me know. 😊

[knpwrs]

How does that look?

[markuswustenberg]

Looks great! Should we add the popovertargetaction now we're at it? :D

[knpwrs]

Done!

[markuswustenberg]

Thank you for your work! 😊