Auto merge of #14493 - shannmu:dynamic_switch, r=epage

feat: Add native comlpetion with CompleteEnv under the nightly

### What does this PR try to resolve?

Related issue #6645
Tracking issue #14520
This PR is the first step to move cargo shell completions to native completions by using `clap_complete` crate. It makes users could complete cargo subcommand and flags.

By using `clap_complete` crate, we could extend the supported shells to Bash, Zsh, Elvish, Fish, and PowerShell. However, at the current stage, the support for PowerShell in `clap_complete` is not fully developed.
See clap-rs/clap#3166 to get more context about what features `clap_complete` has supported.

### How to test and review this PR?

1. Build a test environment, including the necessary short completion scripts, and the `complete` function to start an interactive shell with the help of a pty device and obtain completion results.
2. Simply test the completion results of subcommands in bash, zsh, fish, elvish.

Author: bors

.github/workflows/main.yml

@@ -173,6 +173,13 @@ jobs:
     - run: rustup target add ${{ matrix.other }}
     - run: rustup component add rustc-dev llvm-tools-preview rust-docs
       if: startsWith(matrix.rust, 'nightly')
+    # Install fish, zsh, and elvish only on Ubuntu systems
+    - name: Install fish, zsh, and elvish on Ubuntu
+      run: sudo apt update -y && sudo apt install fish zsh elvish -y
+      if: matrix.os == 'ubuntu-latest'
+    - name: Install fish, elvish on macOS
+      run: brew install fish elvish
+      if: matrix.os == 'macos-14' || matrix.os == 'macos-13'
     - run: sudo apt update -y && sudo apt install lldb gcc-multilib libsecret-1-0 libsecret-1-dev -y
       if: matrix.os == 'ubuntu-latest'
     - run: rustup component add rustfmt || echo "rustfmt not available"
@@ -227,6 +234,7 @@ jobs:
       - run: rustup update --no-self-update stable && rustup default stable
       - run: rustup target add i686-unknown-linux-gnu
       - run: sudo apt update -y && sudo apt install gcc-multilib libsecret-1-0 libsecret-1-dev -y
+      - run: sudo apt update -y && sudo apt install fish zsh elvish -y
       - run: rustup component add rustfmt || echo "rustfmt not available"
       - run: cargo test -p cargo
         env: