[fix] simple theme: make LESS sources available on non build hosts

Include the LESS source files directly in the `.map` files [1].

Drawback: `.map` files grow up in size, but this should not bother, because the
files are only loaded in debug mode.

[1] https://github.com/gruntjs/grunt-contrib-less#outputsourcefiles
[2] https://github.com/searxng/searxng/pull/575#issuecomment-984058868



Suggested-by: @dalf [2]
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>