KYAML is a strict subset of YAML, which is sort of halfway between YAML
and JSON.  It has the following properties:
* Does not depend on whitespace (easier to text-patch and template).
* Always quotes value strings (no ambiguity aroud things like "no").
* Allows quoted keys, but does not require them, and only quotes them if
  they are not obviously safe (e.g. "no" would always be quoted).
* Always uses {} for structs and maps (no more obscure errors about
  mapping values).
* Always uses [] for lists (no more trying to figure out if a dash
  changes the meaning).
* When printing, it includes a header which makes it clear this is YAML
  and not ill-formed JSON.
* Allows trailing commas
* Allows comments,
* Tries to economize on vertical space by "cuddling" some kinds of
  brackets together.
* Retains comments.

Examples:

A struct:

```yaml
metadata: {
  creationTimestamp: "2024-12-11T00:10:11Z",
  labels: {
    app: "hostnames",
  },
  name: "hostnames",
  namespace: "default",
  resourceVersion: "15231643",
  uid: "f64dbcba-9c58-40b0-bbe7-70495efb5202",
}
```

A list of primitves:

```yaml
ipFamilies: [
  "IPv4",
  "IPv6",
]
```

A list of structs:

```yaml
ports: [{
  port: 80,
  protocol: "TCP",
  targetPort: 80,
}, {
  port: 443,
  protocol: "TCP",
  targetPort: 443,
}]
```

A multi-document stream:

```yaml
---
{
  foo: "bar",
}
---
{
  qux: "zrb",
}
```

* Can read 1 file (cmdline)
* Can read multiple files (cmdline)
* Can read stdin
* Can write traditional YAML or KYAML
* Can diff input vs output (-d)
* Can write results to the input files (-w)

It's unclear if we actually need this.  The round-trip test works
without it.

The spec describes escapes for things like space and forward-slash,
which seem wrong to escape, so I may be misreading it.

Merge the "regular" and "compact" tests together.

Add "kyaml" support and yamlfmt