[w3c/clipboard-apis] Specify order of flavors exported to macOS's pasteboard (#137)

When exporting images to the clipboard (via right-click and "Copy Image"), Chrome and Firefox currently order the flavors differently on macOS's pasteboard.

Chrome adds:
```
public.tiff
NeXT TIFF v4.0 pasteboard type
public.html
APPLE HTML pasteboard type
```

Firefox adds:
```
public.html
Apple HTML pasteboard type
public.tiff
NeXT TiFF v4.0 pasteboard type
```

Despite [Apple's Pasteboard Programming Guide](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/PasteboardGuide106/Articles/pbConcepts.html#//apple_ref/doc/uid/TP40008101-SW1) requiring the pasteboard readers to not rely on the order:

"A pasteboard reader must find the data type that best suits its capabilities (if any). Typically, this means selecting the richest type available. In the previous example, a rich text editor might provide RTFD, RTF, and NSString representations of copied data. An application that supports rich text but without images should retrieve the RTF representation; an application that only supports plain text should retrieve the NSString object, whereas an image-editing application might not be able to use the text at all."

some applications, like [Pages](https://en.wikipedia.org/wiki/Pages_(word_processor)) rely on it. See [here](https://bugzilla.mozilla.org/show_bug.cgi?id=1689992).

So for compatibility, the order should be specified.

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