disabled variant in form controls

[GalacticHypernova]

Description

Related: #2907

For many form controls, such as buttons, inputs, checkboxes, radio buttons, text areas, and more, there is no need to have stylistic handling of disabled states by default as they mostly will never reach it.

If we take for example the most common type of online forms, the registration/login form, there is typically an email input and a password input, none of which will ever be disabled. Having the disabled styling on them simply results in more clutter in the HTML and larger payloads, thus reducing performance.

And this doesn't even touch on other use cases of form controls that can never be disabled as per the spec, like using UButton's builtin navigation support when making aesthetic links to other content.

A big performance improvement can be made in this aspect by introducing a new disabled variant for form controls that will be false by default. Only if explicitly set on will it add all styling related to the disabled state.

Additional context

No response

[benjamincanac]

We could get rid of all disabled: classes indeed but this would require a big refactor. I'll try to do this when I have some time!