Add UnitGenerateOptions.Intern for string unit types

[AmanAgnihotri]

I have a use case where I have various string type values and they have low cardinality in the data I'm utilizing. I have to store such data in memory and also do equality checks for such values.

This library is pretty much what I want but with an added UnitGenerateOptions.Intern option, so I can provide it when defining the partial struct and the source generator will intern the string values for those struct types.

Do you think this string interning option can be a useful addition to this library?