[mod] add stylelint process to target themes.simple

Before build, do lint the LESS files using stylelint [1].  The configuration
'stylelint-config-standard' [2] is added to the root of the simple theme [3].

[1] https://stylelint.io/
[2] https://github.com/stylelint/stylelint-config-standard
[3] https://stylelint.io/user-guide/configure



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>