lisp/textmodes/css-mode.el (css--bang-ids): New buffer-local variable
holding the list of bang-rules for the current mode.
(css--font-lock-keywords): Retrieve bang-rules from `css--bang-ids'
instead of computing them.
(css--complete-bang-rule): New function for completing a bang-rule.
(css-completion-at-point): Add support for completing bang-rules.
(scss-font-lock-keywords): Change from a variable to a function in
order to recompute `css--font-lock-keywords' when `css--bang-ids' has
changed.
(scss-mode): Set `css--bang-ids' and recompute font-lock keywords.