Add optional LESS/SCSS vendor auto-prefixing

[dleffler]

Vendor prefixes for compiled .less or .scss files might be desired to alleviate browser anomalies. Recently (6/18/2020) development code was added in Exponent CMS to detect if php-autoprefixer is installed and if so use it. Please be aware the prefixer MUST be installed as below AND this option will add some time for the initial page load when .less/.scss files are rebuilt (esp. bootstrap).

• Download php-autoprefixer https://github.com/padaliyajay/php-autoprefixer and install in /external as folder name 'php-autoprefixer'.
• Download this custom autoload file and place in the /external/php-autoprefixer folder autoload.inc.zip
• Download PHP-CSS-Parser https://github.com/sabberworm/PHP-CSS-Parser and install in /external/php-autoprefixer as folder name 'PHP-CSS-Parser'

To update site stylesheets:

• first clear the CSS cache using the Exponent Toolbar, Super Admin Tools
• turn ON Error Reporting to help force stylesheet compilation (if turned off it won't recompile a .less/.scss file unless the .css file is missing)
• then load the site in your browser, and again it will take longer to (re)compile .less/.scss files

This solution should add vendor prefixes as listed here.

[dleffler]

Added to code and will be included in next release