Allow reading exclude patterns from files (`--exclude-from`)

[jherland]

Commits:

• dir_traversal: Add DirectoryTraversal.exclude_from()
• Settings: Add .exclude_from, paths containing exclude patterns
• test_sample_projects: Run experiments from within the project dir
• test_sample_projects: Add exclude_from tests
• dir_traversal: Prioritize .exclude rules over .exclude_from rules
• Add --exclude-from option to CLI

[jherland]

@mknorps: Thanks to your comments, and having had some time away from this PR, I've realized that this PR is trying to do too much in one go, specifically:

• I agree with you that exclude patterns read from files given by the user should be treated the same as exclude patterns given directly on the command line (see 3e94dfe for beginning to fix this).
• After looking more closely at ruff's respect-gitignore setting, I found two things:
  • (a) The way I handled gitignore files was incomplete: I did not consider global gitignore files (e.g. ~/.config/git/ignore), nor did I auto-detect/parse .gitignore and .ignore files found further inside the project. Really, this cannot be solved with a static .exclude_from set of paths, instead I think we need to detect/parse these files while we are traversing the project structure.
  • (b) The respect-gitignore boolean option is a much more user-friendly way to expose this functionality to the user (rather than manipulating sets of paths). Also, using a simple boolean option frees us up to write a proper implementation for the problems described in (a), rather than somehow trying to combine this into .exclude_from.
• Therefore: --exclude-from should default to the empty set. This ends up simplifying this PR, but adding/implementing the respect-gitignore boolean option will have to be done in a later PR. 🤷🏻‍♂️