Re-evaluate shells included by default

[epage]

When does it make sense for a shell to be included by default?

Considerings

• Levels of support for native completions, Stablize Rust-Native Completion Engine Tracking Issue #3166
• clap_complete::env::Elvish generates code deprecated in 0.18, removed in 0.21 #5729
• Levels of use of the shells Why are the nushell completions not part of the completions crate? #5880 (comment)