`+mcp` structured syntax suffix needs to be registered with IANA

[lrosenthol]

Describe the bug

The +mcp structured syntax suffix needs to be registered with IANA

See https://www.iana.org/assignments/media-type-structured-suffix/media-type-structured-suffix.xhtml

[idosal]

Thanks @lrosenthol ! Please see #59

[liady]

Fixed in #75