supportedstandards in NEP-15 should incorporate NEP-10 formatting detail

[hal0x2328]

Since NEP-10 was marked obsolete once the SupportedStandards was moved to the manifest, there is no current NEP I can find that specifies how the name and number of the NEP should be formatted.

NEP-10 was specific in saying The return values {"nep-5"}, {"NEP5"}, and {"nep5"} will generally be considered invalid responses unless "nep5", for example, is a valid code for a specification in a non-NEP specification system.

NEP-15 does say The supportedstandards field describes the NEP-10 values. but now that NEP-10 is obsolete, maybe the formatting guidelines should be moved into NEP-15.