fix: backend install directory not removed if empty

[roele]

Adds some new functions to remove a directory passing in a list of files to ignore for the empty dir check. This allows to remove the (empty) backend directory despite the new hidden .mise.forge.json file.

Fixes #2017

[roele]

Have experienced some flakiness in the unit tests, even locally they seemed to fail somewhat randomly 🤔

Following was failing ~50% of the time... Don't see how this is related to these changes, maybe it's not.

failures:

---- cli::env::tests::test_env_tiny stdout ----
thread 'cli::env::tests::test_env_tiny' panicked at src/cli/env.rs:117:9:
assertion failed: `(left == right)`

Diff < left / right > :
<export JDXCODE_TINY=1.0.1
>export JDXCODE_TINY=2.1.0


note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    cli::env::tests::test_env_tiny

test result: FAILED. 218 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 5.10s

[jdx]

Hmm, doesn't look related, I'll check it out tomorrow