New command "bazel completion (bash|zsh|fish)"

### Description of the feature request:

Bazel should support printable completions like [kubectl](https://kubernetes.io/docs/reference/kubectl/generated/kubectl_completion/), [helm](https://helm.sh/docs/helm/helm_completion/), [gh-cli](https://cli.github.com/manual/gh_completion), [rustup](https://rust-lang.github.io/rustup/installation/index.html#enable-tab-completion-for-bash-fish-zsh-or-powershell)... I can keep going.
The main cause of delay of bazelbuild/bazelisk#29 although it was the most upvoted issue in bazelisk was that there was no trivial way to actually get it. I believe this shouldn't be the work of bazelisk, but rather a command in bazel itself.

Even if not for the `bazelisk` use case, providing completions directly in the subcommand itself is much more useful, coherent and simpler.

### Which category does this issue belong to?

CLI

### What underlying problem are you trying to solve with this feature?

_No response_

### Which operating system are you running Bazel on?

_No response_

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

The issue names no files, tests, or implementation entry point. First locate Bazel's CLI command definitions and determine how a command can print completions for bash, zsh, and fish; done means the new command produces usable output for all three shells.

Written by the indexing model from the issue text.