Add form-associated element example

[ostomachion]

Add one or more examples of creating a form-associated element using attachInternals() and formAssociated.

https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/attachInternals
https://web.dev/more-capable-form-controls/

[JuanJefry23]

Hello, I would like to take this issue, can you assign this to me ? please.

[bsmth]

Hello, I would like to take this issue, can you assign this to me ? please.

Hi @JuanJefry23 - you don't need it to be assigned, you can let everyone know in this issue if you're working on it and open a pull request when you're ready. Do you understand what needs to be done for the example?