[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>

searx/static/themes/simple/gruntfile.js

@@ -112,8 +112,7 @@ module.exports = function(grunt) {
           ],
           sourceMap: true,
           sourceMapURL: (name) => { const s = name.split('/'); return s[s.length - 1] + '.map';},
-          outputSourceFiles: false,
-          sourceMapRootpath: '../',
+          outputSourceFiles: true,
         },
         files: {
           "css/searxng.min.css": "src/less/style.less",