Add Boolean Return Values to `require_if_theme_supports`

[jeffsebring]

Boolean return values make this function much more useful, saving a call to current_theme_supports in some use cases.

Example:

if ( require_if_theme_supports( 'plugin-theme-magic', __DIR__ . '/includes/plugin-theme-magic.php' ) ) {
   // parents, children, plugins and core getting along really well
}

[jeffsebring]

Patch to add boolean return values to require_if_theme_supports

[DrewAPicture]

fixed docs

[DrewAPicture]

26716.diff​ fixes the docs slightly. Looks good to me.

[SergeyBiryukov]

In 27217:

Add boolean return values to require_if_theme_supports().

props jeffsebring, DrewAPicture.
fixes #26716.