Add Web Share Target API Level 2

[Schweinepriester]

• Spec: https://wicg.github.io/web-share-target/level-2/
• https://www.chromestatus.com/feature/6124071381106688
• https://developers.google.com/web/updates/2019/05/web-share-files

Not to be confused with Web Share Target Level 1 (#4670) nor
Web Share (without Target) Level 1 (https://caniuse.com/#feat=web-share) or Level 2 (#4798).

Maybe once Web Share Target progresses the distinction between levels isnt needed, but at least for Chrome for Android its different releases for Level 1 and Level 2:

• https://www.chromestatus.com/feature/5662315307335680 (71)
• https://www.chromestatus.com/feature/6124071381106688 (76)