Improve the tab character width in the Plugins and Themes editor

[afercia]

In #38667 the Customizer is going to introduce the CSS property tab-size for the Custom CSS textarea. It allows to specify a value, for example an integer for the number of spaces, to represent the width of a tab character.

The Plugins and Themes editor could use this property as well, to enhance the visual presentation of the code indentation within the editor. At the moment Firefox needs the prefixed version and there's no support in IE or Edge.

Screenshot before:

and after setting tab-size: 4:

[afercia]

The proposed patch just sets the tab-size to 4 and removes the outline declaration, since it's already set in forms.css for all the textarea elements.

[afercia]

Refreshed patch: Autoprefixer still wants the -o- prefixed version for Opera 12.1.

[SergeyBiryukov]

In 39897:

Administration: Improve tab character width in Plugins and Themes editor.

Props afercia.
Fixes #38684.