docs(typescript): document packageJsonMergeStrategy

[devin-ai-integration]

Summary

Documents the new TypeScript SDK generator option packageJsonMergeStrategy: 'shallow' | 'deep' (default shallow) on the TypeScript configuration reference, next to packageJson, and adds a pointer from packageJson to it.

Introduced in fern-api/fern#17609. deep lets users add a custom exports condition without redeclaring the generated import/require/default conditions; shallow is the existing behavior where nested objects are replaced wholesale. Should merge after the generator release ships.

Link to Devin session: https://app.devin.ai/sessions/df07e774e6de4b6e9f4a78074bffa093
Open in Devin Desktop: https://app.devin.ai/desktop/session/df07e774e6de4b6e9f4a78074bffa093?variant=devin

[devin-ai-integration]

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[github-actions]

🚫 [vale] <Microsoft.Contractions> _{reported by reviewdog 🐶}
Use 'isn't' instead of 'is not'.

Suggested change

	Removing a generated key (for example dropping the `require` condition) is not expressible under either strategy.
	Removing a generated key (for example dropping the `require` condition) isn't expressible under either strategy.

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1788365635-ts-package-json-merge-strategy.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/sdks/generators/typescript/configuration