Please add supportedFormats property

[WalterYongtaoWang]

For now, implementation of Chromium supports text/palin, image/png, and text/html is on the way (probably more in the future, like image/jpeg etc).
https://bugs.chromium.org/p/chromium/issues/detail?id=931839#c17

We'll need this supportedFormats property (like the video element) to tell which format is supported, so polyfill can fall back to execCommand or other methods.

I'll be really difficult for that without this.

[dway123]

This supportedFormats property seems to be for the navigator.clipboard. While currently all formats supported for read are also supported for write, we may want to consider having some supportedReadFormats and supportedWriteFormats for the sake of flexibility. Additionally, there is a proposal for Raw Clipboard Access, which should presumably support any arbitrary format, so this property should be clear it's intended for sanitized formats (aka the existing Clipboard API).