Core rework

[tingerrr]

This PR reworks the core implementation and comes various improvements.

User facing improvements

• distinguishing of soft and hard errors for the command line
  • exit codes to detect test failures in scripts (relevant for tests: Use typst-test cetz-package/cetz#470)
• improved and more consistent error messages
• more consistent test result reporting
• additional hints for some messages
• correct implementation of --fail-fast
• support for no optimization updates

Internal improvements

• proper separation of test running and result reporting
• less file system checks
• unification of project and file system access

Closes #6.

[tingerrr]

I'm still unsure how I best remove the usages of Reporter in functions of Project, ideally it doesn't need any synchronization because only reporting should be decoupled from the actual operation similar to testing.

[tingerrr]

Things that are not planned for this PR, but I want to note down regardless, as they have become easier to accomplish following this rework:

• improve logging
  • writing logs to log file
  • maybe only display spans on trace level
• make rayon thread pool configurable from the CLI
• handle open crate edge cases
• user configuration
• configure project wide things with convenient setup with typst-manifest using toml_edit

As for now, any further commits should only fix inconsistencies and bugs, I consider the rework finished and this branch ready for testing.

[tingerrr]

I've done various tests locally and in CI over at hydra (don't mind the chaos of commits, CI is hard) and I'm confident that this PR is ready.