convert: improve JSON converter

[kristoff-it]

Right now the JSON converter is using std.json to parse a tree of json.Value but this way we lose line and column information when producing diagnostics.

This means that right now we print error: missing field 'foo', but we can't tell at which line and column of the original json file this field was missing (ie where the surrounding object closes).

To make this happen, we need to switch to a different approach that involves not losing information about the original token stream.

One potentially interesting solution is to start from a JSON token stream and produce a Ziggy AST.

See src/cli/convert.zig for more information.

Also see #17 for the overall goal of completing ziggy convert.

[kristoff-it]

Done in dd6c3cd