feat: add nushell shell support

[MattiasMTS]

• Add Nushell variant to Shell enum with config paths and detection
• Create nushell.nu template with --env flag for cd directive handling
• Add nushell to default shells in install/uninstall commands
• Update completion format to support nushell descriptions
• Add nushell installation to CI workflows
• Document nushell setup in shell integration docs

[max-sixty]

very open to this, thanks for contributing!

we should ensure that it's well-tested, with some moderate share of the tests for existing shells. (we don't have to support all features, but we should be really confident that the ones we do support work well...)

I think it's acceptable to require nushell to be installed on a system in order for --features=shell-integration-tests to pass

[MattiasMTS]

very open to this, thanks for contributing!

we should ensure that it's well-tested, with some moderate share of the tests for existing shells. (we don't have to support all features, but we should be really confident that the ones we do support work well...)

I think it's acceptable to require nushell to be installed on a system in order for --features=shell-integration-tests to pass

Totally agree. I'll try to spend some time nailing this in later this week. Thanks for awesome maintenance and speed on this 🚤