Decoder from io.Reader

Hi, I noticed that this package implements a Decode function that decodes from `[]byte` I think it would be good if there was a function like in the std `encoding` packages with a signature like this: `hcl.NewDecoder(io.Reader) *hcl.Decoder`.

This would align with the established pattern of the `encoding/json` and other packages such as `gopkg.in/yaml`.

So that you could do something like this

```go
dec := hcl.NewDecoder(os.stdin)
var data any
for {
if err := dec.Decode(&data); err == io.EOF {
break
} else if err != nil {
panic(err)
}
}
```

I think if it reads from `[]byte` it's more like `Unmarshal` than `Decode`.

Start by locating the existing Decode function and examining how it consumes []byte. Compare its behavior with Go's encoding/json Decoder and the proposed io.Reader loop, then determine what tests would demonstrate repeated decoding and io.EOF. Done means the package exposes the requested reader-based decoder behavior without breaking the existing API.

Written by the indexing model from the issue text.