Rework and cleanup core implementation

[tingerrr]

The current implementation is a little brittle, this has a lot of downsides:

• error reporting is sometimes done using ?, other times it's swallowed and simply printed to stdout
• some actually likely error cases are not properly handled and result in a simple error message with no hint
• as it currently is, reporting a nonzero exit code on test failure would need an ugly hack
  • this is currently the primary blocker for adoption of typst-test in cetz and in code bases which test using CI in general

The core should be reworked into a solid foundation on which new features can be built. At this stage, the project is still fairly small, and a rewrite is feasible.

I want to achieve the following goals with the rewrite:

• Abstract user facing reporting sufficiently to allow in place progress per test for multiple tests
• Properly handle common error cases better
• Make code easier to follow and reason about
• Make code, which depends on FS operations, easier to test automatically
• Abstract the testing in a way that makes separation into a core library and cli frontend easier down the line