Add command for cycling between CSS color formats

Add command for cycling between CSS color formats

* lisp/textmodes/css-mode.el (css-mode-map): Add keybinding for
'css-cycle-color-format'.
(css--rgb-color): Add support for extracting alpha component.
(css--hex-alpha, css--color-to-4-dpc, css--named-color-to-hex)
(css--format-rgba-alpha, css--hex-to-rgb)
(css--rgb-to-named-color-or-hex): New functions.
(css-cycle-color-format): New command for cycling between color
formats.

* test/lisp/textmodes/css-mode-tests.el (css-test-color-to-4-dpc):
(css-test-named-color-to-hex, css-test-format-rgba-alpha)
(css-test-hex-to-rgb, css-test-rgb-to-named-color-or-hex)
(css-test-cycle-color-format, css-test-hex-alpha): New tests for the
changes mentioned above.

* etc/NEWS: Mention the new command.