Add Full-width Image Feature to Shortcodes

[welpo]

Introduces a new feature that allows images to be displayed at full-width (i.e. equl to header width) inside articles.

Changes include:

• Addition of a new full_width_image shortcode.
• Addition of a new full_width parameter to the image shortcodes. This parameter is optional and defaults to false.
• Addition of necessary CSS for the full-width class to ensure proper display of full-width images.