Description

Nu has behaviour on Windows where, when it can't find an executable on disk, it will "shell out" to cmd.exe. This is because some common shell commands are implemented internally in cmd.exe and do not exist on disk. This PR changes the behaviour so we only shell out for a specific set of known internal commands: ASSOC, DIR, ECHO, FTYPE, MKLINK, START, VER, VOL

Deets

This change is motivated by 2 considerations:

1. cmd.exe's error messages are not as nice as Nu/Miette errors
2. Shelling out to cmd.exe makes it difficult to programmatically determine when a command could not be found at all (example)

I read through the full list of internal commands, and determined that those are the only ones likely to be useful in Nu. Here's a spreadsheet with reasoning for each command.

Prior to this PR:

After this PR:

Tests

Make sure you've done the following:

• Add tests that cover your changes, either in the command examples, the crate/tests folder, or in the /tests folder.
• Try to think about corner cases and various ways how your changes could break. Cover them with tests.
• If adding tests is not possible, please document in the PR body a minimal example with steps on how to reproduce so one can verify your change works.

(no tests added - I think our existing tests cover this)

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra to check that all the tests pass