add new env vars MM_MODULES_DIR and MM_CUSTOMCSS_FILE … (#3530)

… for setting these things from outside (and overriding corresponding
config.js properties `config.foreignModulesDir` and `customCss`)

- remove elements from index.html when loading script or stylesheet
files fails
- removed `config.paths.vendor` (could never work because `vendor` is
hardcoded in `index.html`) and renamed `config.paths.modules` to
`config.foreignModulesDir`. The `config.paths. ...` properties were
implemented in the initial commit in `js/defaults.js` but were never
functional.
- fixes `app.js` which didn't respect `config.paths.modules` before
- as `modules/defaults` is directly set in many places in the source
code restrict `config.paths.modules` to foreign modules (it has never
worked for default modules), now renamed to `config.foreignModulesDir`
- adds new `/env` section in `server.js` for getting the new env vars in
the browser
- fixes TODO in `server.js` so test directories are now only published
when running tests

These changes allow changing some main paths from outside mm with the
new env vars. You now **can** put all user stuff into one directory,
e.g. the `config` dir:
- `config.js` as before
- `custom.css`
- foreign modules

This would simplify other setups e.g. the docker setup. At the moment we
have to deal with 3 directories where 2 of them (`modules` and `css`)
contains mixed stuff, which means mm owned files and user files. This
can now simplified and leads to cleaner setups (if wanted).

CHANGELOG.md

@@ -12,6 +12,8 @@ _This release is scheduled to be released on 2024-10-01._
 ### Added
 
 - [core] Add spelling check (cspell): `npm run test:spelling` and handle spelling issues
+- [core] removed `config.paths.vendor` (could not work because `vendor` is hardcoded in `index.html`), renamed `config.paths.modules` to `config.foreignModulesDir`, added variable `MM_CUSTOMCSS_FILE` which - if set - overrides `config.customCss`, added variable `MM_MODULES_DIR` which - if set - overrides `config.foreignModulesDir`
+- [core] elements are now removed from index.html when loading script or stylesheet files fails
 
 ### Removed