feat: add menu link spacing setting for desktop navigation

[jjroelofs]

Linked issues

• Add setting for desktop main menu link spacing #756

Solution

Add a new theme setting to customize the horizontal spacing (padding) between desktop main menu links.

New setting: menu_link_spacing under DXPR Theme Settings > Header > Main Menu Links

• Type: Range slider
• Range: 0-50 pixels
• Default: 10px (matches Bootstrap's nav-link padding)
• CSS Variable: --dxt-setting-menu-link-spacing

Files changed:

• features/sooper-header/header-theme-settings.inc - Added form element
• features/sooper-settings/css-variables-theme-settings-css.inc - Registered CSS variable
• config/install/dxpr_theme.settings.yml - Added default value
• dxpr_theme_STARTERKIT/config/install/dxpr_theme_STARTERKIT.settings.yml - Added default value
• scss/components/dxpr-theme-header--top.scss - Applied CSS variable to nav links

CSS applied to:

.nav > li > a,
.nav > li > span.nav-link {
  padding-left: var(--dxt-setting-menu-link-spacing, 0.625rem);
  padding-right: var(--dxt-setting-menu-link-spacing, 0.625rem);
}

Checklist

• I have read the CONTRIBUTING.md document.
• My commit messages follow the contributing standards and style of this project.
• My code follows the coding standards and style of this project.
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Need to run update.php after code changes
• Requires a change to end-user documentation.
• Requires a change to developer documentation.
• Requires a change to QA tests.
• Requires a new QA test.
• I have updated the documentation accordingly.
• All new and existing tests passed.