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.

1 files changed, 3 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index ea4a657cba9..bd25f43ad06 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -263,6 +263,9 @@ Can be controlled via the new variable 'footnote-align-to-fn-text'.
 formats (e.g. "black" => "#000000" => "rgb(0, 0, 0)") has been added,
 bound to 'C-c C-f'.
 
+---
+*** CSS mode, SCSS mode, and Less CSS mode now have support for Imenu.
+
 ** SGML mode
 
 ---