Add support for letter-spacing in typography settings

Problem
Currently, the Artisan module allows customization of typography properties like font-size, font-weight, and line-height, but does not support the letter-spacing property. This limits the ability to adjust text spacing directly from the UI.

Solution Implemented

• Added letter-spacing as a configurable option in ArtisanCustomizations.php within getDefaultDefinition().
• Introduced the CSS variable --theme-letter-spacing in getAttachmentStyles() so it can be dynamically applied.
• Updated SCSS files (_typography.scss, _body.scss, _headings.scss, _forms.scss) to use var(--theme-letter-spacing, normal); ensuring consistent application across all text elements.

Testing Conducted
Letter-spacing configuration

• Tested with different values such as 1px, 2px, and 0.5em through the UI.
• Verified that --theme-letter-spacing was correctly generated in the CSS.
• Checked that letter-spacing is applied to body, headings (h1-h6), paragraphs (p), and links (a).