Merge STYLE_DEBUG with SCRIPT_DEBUG

[nacin]

Defining STYLE_DEBUG by itself does not serve concatenated development scripts, because both scripts and styles depend on the same concatenation global (and overriding CONCATENATE_SCRIPTS constant).

It will serve the development version of colors, but that's about it.

I suggest we get rid of STYLE_DEBUG and change all of its references to SCRIPT_DEBUG.

The alternative would be to allow concatenation to be controlled independently, but that would introduce another global and change existing behavior, having an effect outside of debugging. Removing STYLE_DEBUG has no real downside and makes it one less constant for developers to define.

[nacin]

Likewise, because of the $concatenated_scripts bottleneck, setting only SCRIPT_DEBUG will unconcatenate styles, but leave them minified.

[nacin]

(In [13183]) Remove STYLE_DEBUG constant, SCRIPT_DEBUG now handles both scripts and CSS. Fixes #12237

[matveb]

Shouldn't wp-admin/css/ms.dev.css be included when SCRIPT_DEBUG?

[nacin]

#12292