Add platform-provided mixins explainer

[anaskim]

V1 includes:

• Rewording on interoperability.
• Adds examples on current workarounds used by frameworks.
• Adds goal to clarify the general and specific objectives of the proposal.
• "behavior" -> "mixin", to avoid confusion with "behaviour" spelling.
• Simplifies to add mixins to attachInternals() and avoid dynamic changes at runtime.
• Adds mixins property to ElementInternals to query behaviors.
• Removed "Open questions" section and expanded "Future Work" section with a "collision handling" note and 2 more proposed mixins (input and table behaviors).
• Updated "Accessibility, Security, and Privacy Considerations" section.
• Updated acknowledgements.

[KurtCattiSchmidt]

Another pro to this approach is that it allows for child classes to easily extend from mixins. e.g.

class CustomSubmitButton extends HTMLSubmitButtonMixin(HTMLElement) { ... }
class ExtraCustomSubmitButton extends CustomSubmitButton {...}

Is this possible with the current mixin proposal?

[KurtCattiSchmidt]

Maybe rephrase this as "Limited Shadow DOM support - only certain elements can be shadow hosts" and link to https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow#elements_you_can_attach_a_shadow_to

[KurtCattiSchmidt]

Link this to https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow#elements_you_can_attach_a_shadow_to

[KurtCattiSchmidt]

"platform behaviors" is a bit vague, maybe say "platform behaviors for built in elements"

[KurtCattiSchmidt]

Does it make sense to list "Extend Markup Like Customizable Select"? Or is that too different of a problem? The dealbreaking con with this approach is that it won't work for all types (or any types beyond select?).

https://developer.mozilla.org/en-US/docs/Learn_web_development/Extensions/Forms/Customizable_select