[HTML] Add support for dirname attribute

[bsmth]

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
• Add entry to Firefox experimental features page if feature is not yet enabled in release

Features to document

"The dirname attribute on a form control element enables the submission of the directionality of the element, and gives the name of the control that contains this value during form submission. If such an attribute is specified, its value must not be the empty string."

<form action="addcomment.cgi" method=post>
 <p><label>Comment: <input type=text name="comment" dirname="comment.dir" required></label></p>
 <p><button name="mode" type=submit value="add">Post Comment</button></p>
</form>

From release notes:

Added support for the dirname attribute to pass text directionality information about input and textarea elements to the server.

Related Gecko bugs

• https://bugzilla.mozilla.org/show_bug.cgi?id=675943

Other

• Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.
• Check if glossary updates are required for the feature you're documenting - whether an existing term needs to be updated or a new term should be added.
• Check if BCD update means that content pages need to have experimental markup removed or deprecated markup added (front matter tags and macros).

Related issues and pull requests:

• BCD https://github.com/mdn/browser-compat-data/pull/20109/files
• Relnote: relnote(116): support dirname on textarea and input elements #28032

[bsmth]

Attribute	Element(s)	Description	Value
dirname	input; textarea	Name of form control to use for sending the element's directionality in form submission	Text*

[bsmth]

Closing for now as basic details have been added & release note. Improvements are incoming with attribute page & more information on input types that are being supported soon:

If the element has a dirname attribute, that attribute's value is not the empty string, and the element is either a textarea element or an input element whose type attribute is in the Hidden, Text, Search, Telephone, URL, or Email state, then:

[bsmth]

See also: #28146