supports fish shell - v2

[skshetry]

On top of #174. Closes #174.

Closes #134.

• Fixes subcommands and nested commands to work properly, limited upto 3 levels. (eg: flags/subcommands beyound rustup toolchain help install won't be autocompleted)
• Use description/help for subcommands properly (it was not working for aliased subcommands).

Both were reported in https://github.com/iterative/shtab/pull/174#issuecomment-2483432400.

• Also added support for choices.

See soraxas/feat-support-fish-shell..skshetry/fish-shell-v2 for the change in this PR.

Builds upon the ideas from https://github.com/clap-rs/clap/pull/5568.