Scope extensions explainer: accept multiple scope values to allow configuration for different platforms

[LuHuangMSFT]

scope_extensions should accept multiple scope values and have the client pick the first that's compatible with the current platform.

[LuHuangMSFT]

@alancutter

[alancutter]

From an external discussion:

I think scope extensions will need to grapple with the technical reality that different platforms have different capabilities for URL matching.
We can either:
A) Attempt to perform lossy conversions on the client side.
B) Restrict the spec to the lowest common denominator.
C) Accept multiple scope values and have the client pick the first that's compatible with the current platform.

I would like to see us do C as it lets developers handle less capable platforms themselves instead of A; us having to guess what lossy conversion will work for everyone or B; sites losing out on available capabilities.

[alancutter]

We can follow display_override's example; accept a list and use the first value that's supported.