Redmine#6008: provide Mustache % and $ escape sequence for serialization

Summary:

```
all variables serialized, multi-line format: {{%vars}}

all classes serialized, compact format: {{$classes}}

a single variable serialized: %focus = {{%vars.main.focus}}

a single variable serialized: $focus = {{$vars.main.focus}}
```

Test policy:

```
bundle agent main
{
  vars:
      "focus" string => "attention";

  files:
      "/tmp/out.json"
      create => "true",
      edit_template => "$(this.promise_filename).mustache",
      template_method => "mustache";
}
```

Unlike writing the output of `storejson()` or `format("%S")` to a file, this doesn't go through the evaluator strings.

Please ignore the acceptance test, it's not ready yet.  I will finish it if the proposed functionality is acceptable.