Support value validations

It'd be useful to be able to attach a set of validations that are run on a returned value.

Might either (or both) be an extra argument on all the ConfigReader methods, or be part of the imagined property macros.

Would be good to provide common validations built-in, plus a way to combine them (and + or), but also allow packages to add more.

Start by reading the ConfigReader methods mentioned in the issue and any existing property-macro design or API documentation in the package. Define the validation model, built-in rules, composition behavior, and extension point before implementation; done means the API and its expected validation semantics are agreed and covered by tests.

Written by the indexing model from the issue text.