Parse #[ts(tag = "...")] on structs

[escritorio-gustavo]

This PR adds a #[ts(...)] equivalent to #[serde(tag = "...")] on structs

[NyxCode]

Ohh, right, we've already got tag on StructAttr since we use that for enums under the hood. Very elegant, love it!

[escritorio-gustavo]

Ohh, right, we've already got tag on StructAttr since we use that for enums under the hood

I don't think that's why we already had tag on StructAttr, since EnumAttr has its own tag to handle that. Instead we did already support the serde feature of adding a tag to a struct, but it required serde-compat and #[serde(...)]