TypeScript guidelines: Add Derive types from authoritative sources guideline

[MajorLift]

Adds a Derive types from authoritative sources instead of re-declaring them subsection to the Type Inference section.

The section already covers preferring inference for values and avoiding accidental widening, but not the common case (especially with AI) of hand-writing a type that restates one an authoritative source already defines.

Includes counterexamples.

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Note

Low Risk
Documentation-only change to docs/typescript.md; no runtime or build behavior is affected.

Overview
Adds a Type Inference subsection, Derive types from authoritative sources instead of re-declaring them, to docs/typescript.md.

It tells contributors to derive types from existing sources (controller state, returns, @metamask/* exports, schemas) via State['field'], typeof, ReturnType/Parameters, and Pick/Omit/Partial, instead of hand-written duplicates that widen shapes and drift from the source. It links to the existing widening and inference guidelines and includes a permalinked counterexample: getMetaMaskState: () => Record<string, unknown> versus composing from TokenListState to avoid per-use-site casts and as never.

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