Add color highlighting to css-mode

Add color highlighting to css-mode

Bug#25525
* lisp/textmodes/css-mode.el (css--color-map): New constant.
(css-value-class-alist): Use css--color-map.
(css--number-regexp, css--percent-regexp)
(css--number-or-percent-regexp, css--angle-regexp): New constants.
(css--color-skip-blanks, css--rgb-color, css--hsl-color): New
functions.
(css--colors-regexp): New constant.
(css--hex-color, css--named-color, css--compute-color)
(css--contrasty-color, css--fontify-colors)
(css--fontify-region): New functions.
(css-mode): Set font-lock-fontify-region-function.
(css-mode-syntax-table): Set syntax on more characters.
(css-fontify-colors): New defcustom.
(scss-mode-syntax-table): Define syntax for ?$ and ?%.
* test/lisp/textmodes/css-mode-tests.el (css-test-property-values):
Update.
(css-test-rgb-parser, css-test-hsl-parser)
(css-test-named-color): New tests.
* etc/NEWS: Add entry.