Adds a Yaml specification generator for another command line completion
tool, `carapace`.

https://carapace.sh/

The benefit with Carapace, compared with Completely, is that it supports
a lot of shells, including Bash, ZSH, Fish, Elvish, Powershell, Cmd, and
more.

The drawback is that `carapace` requires end users to install it, rather
than just distributing a pre-baked script with the command.

Example of a carapace Yaml spec file:
```yaml
# yaml-language-server: $schema=https://carapace.sh/schemas/command.json
name: example
persistentFlags:
  -q, --quiet: Suppress all cli output. Is overridden by  -v, --verbose.
  -v, --verbose: Prints additional information useful for development. Overrides --q, --quiet.

commands:
  - name: completion
    flags:
      -t, --target=: 
      -e, --exec-name=: Override the name of the executable
      -f, --file=: Write the specification to a file instead of stdout
    completion:
      flag:
        target: ["completely", "carapace"]
        file: ["$files"]

  - name: show
    flags:
      -i, --interval=!: The time interval
      -n, --multi=*: The multioption
      -m, --mandatory: The mandatory flag
      --no-mandatory: The mandatory flag
    exclusiveFlags:
      - [mandatory, no-mandatory]
    completion:
      flag:
        multi: ["a", "b", "c", "d", "e", "f", "g"]
```

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- New Features
  - Experimental command-line completion support.
  - Generate completion specs for Carapace and Completely.
- Supports overriding executable name and writing output to a file or
stdout.

- Documentation
- Added “Command line completion” docs with setup, generation, and
installation steps for Bash and Zsh.
  - New detailed guide covering both Carapace and Completely workflows.

- Tests
- Added end-to-end tests for completion command visibility and spec
generation across targets and output modes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

To make it easy for command developers to distribute the shell
completion scripts to their end users, this adds two sub-commands:

- `completion embed`: Embeds a completion script in a dart source file
for inclusion in the command itself.
- `completion install`: Installs an embedded completion script in the
user's environment.

### Example

This PR also includes a complete example in the `example` folder of
using the completion feature with embedding and installing.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Enable command-line completion via a new non-experimental flag and
pass embedded completion scripts to the runner.
- Add commands to generate, embed, and install completion scripts for
multiple targets (Completely, Carapace), plus install workflow for end
users.
- **Documentation**
- Major README overhaul with step-by-step installation, usage examples,
generation/embed/install workflows, and an added completion example.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

- cli_tools@0.8.0
 - config@0.8.0

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- Documentation
- Consolidated 0.8.0 release notes by merging beta entries into a single
stable section.
- Clarified included items (e.g., command-line completion, Dart 3.9
testing) and removed obsolete experimental/revert notes.
- Chores
  - Promoted packages from 0.8.0-beta.1 to 0.8.0.
  - Aligned dependency versions to stable 0.8.0.
  - No runtime or behavior changes.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->