Also: let the theme service not change classes on document.body but the actual container of the workbench

adopt new loader, make sure each loader instance has a static seed so that filenames don't conflict, #63605

* bootstrap  - first cut

* bootstrap => bootstrap-fork, bootstrap-shared => bootstrap

* bootstrap - extract uriFromPath()

* bootstrap - extract readfile/writefile

* bootstrap - share more code when loading renderers

* boostrap-electron-browser => bootstrap-window

* bootstrap - cleanup main.js

* bootstrap - handle index.js

* bootstrap 💄

This reverts commit 57ab2365.