RTLCSS should swap Dashicons left/right arrows

[ocean90]

Instead of adding a .rtl case each time we reference a rigt or left arrow from Dashicons RTLCSS should handle this.

[ocean90]

In 31577:

Press This: RTL improvements.

• Load the RTL version of press-this-editor.css.
• Use more semantic class names for the arrows, Makes it easier to swap the arrows in RTL, see #31478.

see #31474.

[ocean90]

In 31579:

Build: Let RTLCSS handle swapping the codes for right/left arrows from Dashicons.

Exclude wp-includes/css/dashicons.css from core task since we don't want/need a RTL version of it.

fixes #31478.