add getJsonSchema() to TypeChatJsonValidator

[mmoskal]

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This is a little experiment, to possibly allow using structured outputs with TypeChat.

Note that you would want to only use this as a grammar for constraining the output, not pass the schema to the model - the TypeScript file is probably much better for this, as the schema is 3x bigger than TypeScript source (with indentation; without indentation schema is 1.5x bigger). OpenAI-compatible endpoints will often pass the schema to the model using chat template.

This works for all example schemas (excluding Zod, but it can be handled natively I guess) and all schemas in TypeAgent.