MDL-75012 js: Put shifter on a diet

This commit switches to a new custom version of Shifter which sees it
put on a massive diet.

Many of the standard Shifter features are totally unused by Moodle, but
they add dependency spaghetti. Some of those dependencies will start to
cause us issues as they have not been updated for a very long time, are
no longer maintained, and are no longer compatible with anything
resembling a recent version of NodeJS.

package.json

@@ -38,7 +38,7 @@
     "postcss-scss": "^4.0.6",
     "rollup-plugin-terser": "^7.0.2",
     "semver": "^7.3.8",
-    "shifter": "https://github.com/andrewnicols/shifter/releases/download/v1.2.0_moodle-1/shifter-1.2.0_moodle-1.tar.gz",
+    "shifter": "https://github.com/andrewnicols/shifter/releases/download/v1.2.0_moodle-slim-0/shifter-1.2.0_moodle-slim-0.tar.gz",
     "stylelint": "^14.16.1",
     "stylelint-csstree-validator": "^2.1.0",
     "xmldom": "^0.6.0",