Fix the text editor styles after the mixins reset

[youknowriad]

Related #14509 (comment)

This turns out to be a little bit more complex that I originally thought.
I think it just surfaces the point I raised in #14509 saying that we should try to kill all "resets" in the future in favor of styles per components.

The problem is that the refactoring in #14509 applied the textarea and input resets more globally which means we need to reset these styles in the textarea of the text editor. The problem is that the specificity is higher for the reset because we can't apply it globally to the page (admin menus...), this uses !important to override these styles.

[jasmussen]

I think it's okay to use the !importants here, and I agree with your ultimate goal. The CSS bleed from the rest of the wp-admin styles is also the underlying root cause behind #14407.

However I'm seeing this:

It used to be a code font, and the font size appears too small. Maybe those need !important's too? And the focus style for the input field, did it used to be darker? And should the text editor even have the new left-border-heavy styles that blocks have? @kjellr

If this is an urgent PR, it is okay with me to ship this once the font and font size has been fixed. 👍 👍 — but then we need to look at the border-color/thickness separately.

Thanks Riad.

[kjellr]

And should the text editor even have the new left-border-heavy styles that blocks have? @kjellr

Probably not. It looks like it's applied to just the page title. I can remove that in a separate PR. 👍

@youknowriad I left one tiny note — aside from that this is good to go.