Fixes and tweaks for the new Less CSS mode

Fixes and tweaks for the new Less CSS mode

* etc/NEWS: Add an entry for the new mode.

* lisp/textmodes/less-css-mode.el (less-css): Tweak docstring.
(less-css-lessc-command): Tweak docstring. Don't mark it as
safe. Don't autoload.
(less-css-compile-at-save, less-css-lessc-options)
(less-css-output-directory): Tweak docstrings. Don't autoload.
(less-css-output-file-name): Tweak docstring. Don't mark it as safe.
(less-css-input-file-name): Tweak docstring. Don't autoload.
(less-css-compile-maybe): Use `when' for one-armed `if'.
(less-css--output-path): Tweak docstring.
(less-css--maybe-shell-quote-command): Remove function.
(less-css-compile): Don't autoload. Tweak docstring and message. Fix
compiler warning. Use `string-join' instead of `mapconcat'.
(less-css-font-lock-keywords): Use `font-lock-variable-name-face' for
variables.
(less-css-mode-syntax-table, less-css-mode-map): New variables.
(less-css-mode): Change status line mode name from "LESS" to
"Less". Tweak docstring. Move syntax table definitions to
`less-css-mode-syntax-table'.
(less-css-indent-line): Remove function.