Add duckdb::JsonValue

[Maxxen]

This PR adds a new JsonValue class to represent/parse/print json objects.
JSON is everywhere, and its pretty useful to be able to have a small json utility built into DuckDB when working with extensions or other tooling, even if its not used in the query engine itself, or as efficient as the implementation in the dedicated JSON extension.

Right now this only replaces the ToJSONMap() function and co used when stringifying exceptions in the ErrorData, but I have some plans and ideas to use it in:

• Parsing GeoParquet metadata in the parquet extension
• Moving the json_serialize_sql(), json_parse_serialized_sql() and JsonSerializer functions into core. This would allow use to tighten up the serializer infrastructure a bit if we can assume that extensions won't push their own serializers.
• Converting some of the serialization generation scripts into proper c++ tools as IMO they are starting to outgrow python in complexity.
• As an output format for a future auto-documentation-generation subsystem for sql functions
• Maybe unifying the json output option in the shell?
• Other configs, e.g. in the fuzzer, etc.

The parser does not handle unicode escape characters, but should otherwise be JSON compliant. We can make it more sophisticated in the future.

[Tishj]

Converting some of the serialization generation scripts into proper c++ tools as IMO they are starting to outgrow python in complexity.

I'm not sure I agree with this statement, they are starting to outgrow the current architecture of the script however.
A refactor is needed and I've been thinking of some ways to do it, but I need to become more familiar with all of its current quirks

[Mytherin]

Thanks for the PR! Looks great - some minor comments: