Improve form labels and descriptions for the Tags meta box in the Edit Post screen

[afercia]

Noticed a couple of things that could be improved in the Tags meta box:

• with JavaScript on, the input field label is set to the meta box title and it says just "Tags", maybe it should use the label provided by $taxonomy->labels->add_new_item the default is 'Add New Tag'
• with JavaScript off, a textarea appears instead of the input field. This textarea doesn't have a label at all, the "Add or remove tags" text is just a paragraph
• a nice improvement would be using the description below the form as target for an aria-describedby attribute (note: Press This already does this)

[afercia]

The proposed patch improves the labels and uses aria-describedby attributes to target the description. See screenshot below:

• on the left: with JavaScript on, no visual changes
• on the right: with JavaScript off, added a little more vertical space between the textarea and the description

[afercia]

Refreshed patch to use unique IDs in case there are multiple "tags" (non hierarchical custom taxonomies) meta boxes.

[afercia]

In 35609:

Accessibility: Improve form labels and descriptions for the Tags meta box in the Edit Post screen.

Fixes #34627.