The main motivation behind this is simplicity and correctnes, but the new small config library is also faster:
```
BenchmarkDefaultConfigProvider/Viper-16 252418 4546 ns/op 2720 B/op 30 allocs/op
BenchmarkDefaultConfigProvider/Custom-16 450756 2651 ns/op 1008 B/op 6 allocs/op
```
Fixes #8633
Fixes #8618
Fixes #8630
Updates #8591
Closes #6680
Closes #5192
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 #7644
Fixes #7656
Fixes #7675
