This change is mostly motivated to get a more stable CI build (we're building the Hugo site there, with Instagram and Twitter shortcodes sometimes failing).
Fixes#7866
This commit also introduces a convention where these common JS config files, including `package.hugo.json`, gets mounted into:
```
assets/_jsconfig
´``
These files mapped to their real filename will be added to the environment when running PostCSS, Babel etc., so you can do `process.env.HUGO_FILE_TAILWIND_CONFIG_JS` to resolve the real filename.
But do note that `assets` is a composite/union filesystem, so if your config file is not meant to be overridden, name them something specific.
This commit also adds adds `workDir/node_modules` to `NODE_PATH` and `HUGO_WORKDIR` to the env when running the JS tools above.
Fixes#7644Fixes#7656Fixes#7675
