Complete `ziggy convert`

[kristoff-it]

Related issues

• convert: improve JSON converter #14
• convert: conversion strategies for struct unions #21
• convert: implement YAML support #15
• convert: implement TOML support #16

Additionally support for PATH mode still needs to be implemented, alongside all the other minor flags.

See ziggy convert --help for a full list of features that must be supported.

[tamadamas]

@kristoff-it I'm going to implement all these starting with #16

[seangemini2025]

Would love to see this