Serde support

[devongovett]

Closes #143

This adds serde serialization and deserialization support to all AST nodes, behind a feature flag. Appears to work, but need to verify the output.

cc. @lucacasonato @johno

[devongovett]

Did some testing and verified the output seems reasonable. There are certainly improvements we can make (e.g. a nicer format for vendor prefixes in properties, more detailed serialization of selector components, etc.), but it seems usable for now.

To use, you need to enable the serde feature flag for the crate. You can also test the example in the readme which parses the provided filename and outputs JSON to stdout.

cargo run --example serialize --features serde test.css | jless

The format follows the documented rust structs, but with kebab case for enum variants (matching CSS itself), and a few other minor differences. Not sure if possible, but a future improvement could be to somehow generate TypeScript definitions for the format.

Going to merge as MVP. Please let me know if you have feedback!