docs: Add section on type-only dependencies

[rekmarks]

Adds a section explaining when you should list a type-only dependency as a dev vs. prod dependency. Also opportunistically fixes header hierarchy in TypeScript docs.

---

Note

Adds a new "Type-Only Dependencies" section and normalizes heading hierarchy in docs/typescript.md.

• Docs (TypeScript Guidelines):
  • New Section: Adds "Type-Only Dependencies" explaining when a type-only import should be a dev vs. prod dependency, with link to the TypeScript handbook.
  • Heading Hierarchy: Promotes multiple subheadings from #### to ### for consistency across Generic Types, Interfaces, Enums, and Functions sections.

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[jiexi]

Suggested change

	If package `a` imports only types from `b`, `b` be a dev or production dependency of `a`?
	If package `a` imports only types from package `b`, should `b` be a dev or production dependency of `a`?

[rekmarks]

Going with @mcmire under the logic that it's clear from the context that b is a package.

[MajorLift]

Linking for reference:

• Previous discussion on this topic: Categorize type-only and controller-messaged deps correctly core#4449
• Existing guidelines on type-related breaking changes: https://github.com/MetaMask/core/blob/main/docs/reviewing-release-prs.md#breaking-changes

[MajorLift]

The current header hierarchy was intended to keep guideline entries consistently at H4, and to leave room for sections to be organized with H3 sub-headings (as e.g. Types already is).

The entries that have been promoted to H3 are now at the same level as sub-chapter headers e.g. Type Inference. Could this be potentially confusing for readers and future editors? Not necessarily opposed to the change, as it is in keeping with markdown lint rules.

[mcmire]

LGTM!

[rekmarks]

@MajorLift I think it's aesthetically frustrating more than anything else, but happy to revert if anyone feels strongly about it.