feat: add `resolveConfigPath` to return resolved config file path

[danielroe]

https://github.com/nuxt/test-utils/pull/1381#discussion_r2268785522

this is a first stab at extracting logic for resolveConfigPath. I've left all behaviour untouched and still running in the same order, just pulled out a couple of routines which seemed relevant for resolving the source file - which means that there's probably scope for simplifying this

I've also switched to explicit exports for index.ts but of course that could be reverted