Improve error messages when passing the wrong type to a known bsconfig key

Right now bsb uses a way to parse the `bsconfig.json` that doesn't error on keys that are of the wrong type (at least not for all fields). We explicitly do it for some but not others.
We should do that.

Start by locating the parser for bsconfig.json in bsb and reviewing how known keys are currently validated. Compare the fields that already reject incorrect types with those that do not; done means every known key consistently reports an error when given the wrong type, with coverage added for the affected cases.

Written by the indexing model from the issue text.