let start open anything and everything (#7580)

# Description

Fixes #7546 's request. I'm unsure, so hopefully someone in charge of
design can chip in.

# User-Facing Changes

`open` now opens directories in the default file manager.

# Tests + Formatting

Don't forget to add tests that cover your changes.

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 -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/default_context.rs

@@ -275,6 +275,7 @@ pub fn create_default_context() -> EngineState {
             Mkdir,
             Mv,
             Open,
+            Start,
             Rm,
             Save,
             Touch,