Feature request: test selection

Test frameworks for other languages often have a way to filter which test are run, which is useful for iterating on a single test without having to run every test. Examples are [cargo test](https://doc.rust-lang.org/cargo/commands/cargo-test.html), [python's unittest](https://docs.python.org/3/library/unittest.html#command-line-interface), [mix test](https://mix.hexdocs.pm/1.20.2/Mix.Tasks.Test.html), and [unitest](https://unitest.hexdocs.pm/). While gleeunit intends to be a simple test runner, and in the past you've expressed a lack of interest in new features, I think a way to filter based on filename or test name is simple enough and extremely useful for iterating on core packages that it would be a good fit for gleeunit.

My proposal is to add two new command line switches, `--include` and `--exclude` that each take a list of function names to include or exclude. To keep things simple, each switch can only be used once, and specifying a test in both include and exclude will cause an error:

Example usage:

```sh
gleam test # run all tests
gleam test --include foo_test # only run foo_test
gleam test --include foo_test bar_test # only run foo_test and bar_test
gleam test --exclude baz_test # run all tests except baz_test
gleam test --include foo_test bar_test --exclude bar_test # error! bar_test included and excluded
```

This should be simple to implement without adding any complexity to the existing code, and can be done without any dependencies outside of gleam_stdlib. If you're interested in this feature I'd be happy to work on it!

Start at the `gleam test` command and trace how test names are discovered and selected by gleeunit. Implement the single-use `--include` and `--exclude` switches, reject tests named in both lists, and verify that the examples run the expected subset.

Written by the indexing model from the issue text.