Add missing diagnostic message for private method quick fix

[drtootsie]

Fixes #37782

Summary

The code fix logic for adding missing private methods already existed (code 90038), but the diagnostic message was missing from diagnosticMessages.json.

Changes

Added the diagnostic message "Declare private method '{0}'" with code 90038 to enable the quick fix to appear in the IDE.

Testing

The fix can be verified by:

1. Building the compiler: npm install && npm run build
2. Creating a test file with a missing private method reference
3. Confirming the quick fix appears in the IDE

[jakebailey]

I'm not sure how this would work, our diagnostics generate code which we then reference, if this were all that was needed, the code would not be compiling today.

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

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