Fix svg-mixer support for sub-directories

By default, svg-mixer only uses the SVG's file name as its ID. If any SVG files are stored in sub-directories, that information is ignored in the assembled spritesheet. This is problematic since context is lost (the sub-directory's name) and risks duplicate symbol IDs.

This commit introduces a new NPM dependency, `common-path` to resolve the longest common base path, and uses a dependency of `svg-mixer` named `url-slug` in order to prefix the directory name.

package.json

@@ -22,6 +22,7 @@
     "devDependencies": {
         "autoprefixer": "^10.4.17",
         "browser-sync": "^3.0.2",
+        "common-path": "^1.0.1",
         "concat": "^1.0.3",
         "esbuild": "^0.20.0",
         "kleur": "^4.1.5",