Rename remaining references of "selected" to "highlighted"

[westonruter]

This PR amends #92 to further fix #84.

Also:

• Use background instead of bg.
• Disambiguate get_option() function from core by renaming to get_plugin_option(). Also prevent non-existent option from causing PHP error.
• Reduce syntax_highlighting_code_selected_line_bg filter to syntax_highlighted_line_background_color.
• Remove unused shcb-selected-lines class name.
• Fix insertion of styles when not in development mode.

[allejo]

I added f67b59d because this was the second time we had forgot to update our transient rendering 😅

[westonruter]

I added f67b59d because this was the second time we had forgot to update our transient rendering 😅

I just realized that I did the same essentially in 36f536f as part of #97.