Implement `serde::Serialize` on AST

[lucacasonato]

This would be very useful for the CSSOM implementation in Deno (also see #5). Many cssparser structs already have serde::Serialize implementations when you enable the serde feature for cssparser.

If the binary size increase is too much, we can flag it with a crate feature.

I'll try to open a PR later this week.

[johno]

Somewhat relatedly, it'd be great if the properties themselves implement serde::Serialize. For my use case I want to run the parser on property values and get that data back via napi in node.

Is this a feature that folks would accept in this project? If not, is there a recommended way to serialize this data without (hopefully) having to implement Property and all its variants?

[devongovett]

Yep, serde support would be great!

[devongovett]

Assuming you didn't start this @lucacasonato. If not I can probably do it this weekend.

[lucacasonato]

@devongovett I did start on this actually - I won't be able to push it up until Tuesday unfortunately though (the branch is on a different device).