using the file ext caddyfile for a config file does not work

```
./caddy run  -c test.caddyfile
2024/05/31 14:25:55.237 INFO    using config from file  {"file": "test.caddyfile"}
Error: config is not valid JSON: invalid character ':' looking for beginning of value; did you mean to use a config adapter (the --adapter flag)?
```

test.caddyfile
```
:2015
respond "Hello, world!"
```