outputparser: improve DefinedOutputParser

[amitaifrey]

The DefinedOutputParser prompted the LLM with a Typescript schema, but expected a json in response: https://github.com/tmc/langchaingo/blob/1975058648b5914fdd9dc53434c5b59f219e2b5c/outputparser/defined.go\#L65-69 Now it also requests a json.

PR Checklist

• Read the Contributing documentation.
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• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
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[tmc]

oi, thanks.