Add Imenu support to CSS mode and its derivatives

Add Imenu support to CSS mode and its derivatives

* lisp/textmodes/css-mode.el (css--join-nested-selectors)
(css--prev-index-position, css--extract-index-name): New helper
functions for supporting Imenu.
(css-mode): Set `imenu-space-replacement',
`imenu-prev-index-position-function', and
`imenu-extract-index-name-function'.
(css-current-defun-name): Reuse `css--prev-index-position' and
`css--extract-index-name' to support nested selectors.

* test/lisp/textmodes/css-mode-tests.el (css-test-current-defun-name):
Fix character index.
(css-test-join-nested-selectors): New tests for
`css--join-nested-selectors'.

* etc/NEWS: Add news entry.