[Feature] Add file loading helper functions

[Aderinom]

One sentence description of the feature you would like to request

Add some small helpers to load config files.

Describe the solution you'd like

const cfg = new ConfigBuilder<...>()
              .loadFile("config",opts)

Describe alternatives you've considered

Users can also just use import "./config.json" for example.
However i guess this will be something people might ask for.

Furthermore this would allow us to use e.g. https://www.npmjs.com/package/json5?activeTab=readme
to have a less whiny json parser

Additional context

Once again. I'd like JSON5 to be an optional dependency the user can Opt-in to use.