[img-conversion] canDecodeType \ canEncodeType could be "supports" (#12)

I'm wondering, why not: 
```
partial interface ImageData {
  static CanDoOperation supports (EncoderOpts options);
};
enum CanDoOperation{
  "", "maybe", "probably"
};
enum Operation {
  "encode", "decode"
};
```

Then:
```
var ops = {operation: "decode", type: "image/jpeg"};
if (ImageData.supports(ops)){
    //do thing..
}
``` 



---
Reply to this email directly or view it on GitHub:
https://github.com/WICG/img-conversion/issues/12