Fix incorrect use of add_theme_support( 'editor-style(s)' )

[Soean]

In the functions add_editor_style(), remove_editor_styles() and remove_theme_support() the Theme $feature parametereditor-style is used instead of editor-styles.

It should be: add_theme_support( 'editor-styles' );

See docs: ​https://developer.wordpress.org/block-editor/how-to-guides/themes/theme-support/#editor-styles
See code docs: ​https://github.com/WordPress/wordpress-develop/blob/8d0aed455b1790c5d51386f7675d9e8d68e48edf/src/wp-includes/theme.php#L2672

Trac ticket: https://core.trac.wordpress.org/ticket/60826

This will require a Unit test update, too.

[Soean]

Introduced in https://core.trac.wordpress.org/ticket/14903

@audrasjb I updated the tests and added a new test for editor styles.

[sabernhardt]

Related/duplicate: #51755

[oglekler]

I wonder if this can possibly break something for people who used 'editor-style' because it is used in the core? 🤔

add_editor_style() function declaring support for 'editor-style' and I found this confusing, so I added "add_theme_support( 'editor-styles' )" before add_editor_style() in theme I am working on (classic approach works for me better).

I also wonder if test should count on different state for classic and block themes, because for block themes, 'editor-styles' declared by default.

I think, the patch needs rebase.

[hellofromTonya]

Is the block editor using these functions?

These functions are intended and are documented for the TinyMCE (see [13441] and [16721]).

For example, add_editor_style() is ​documented as:

Adds callback for custom TinyMCE editor stylesheets.

I'm curious what the impact might be of changing these instances and if any shims might be needed for older and non-block themes using them.

[oglekler]

Because we need further investigation and clearly a way forward, I am moving this to the next milestone.