refactor: add .js import extensions to Assets packages

[Mrtenz]

Summary

This PR adds .js import extensions to all relative imports in the Assets
packages, in preparation for the ESM refactor.

The ESLint rule that enforces this (n/file-extension-in-import +
import-x/extensions) has been added for these packages in
eslint.config.mjs.

Packages

• @metamask/assets-controller
• @metamask/assets-controllers
• @metamask/network-enablement-controller

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Note

Low Risk
Mechanical import-path and tooling updates only; no auth, data, or runtime behavior changes, though the large touch surface means build/test breakage if any path was missed.

Overview
Prepares @metamask/assets-controller, @metamask/assets-controllers, and @metamask/network-enablement-controller for ESM by appending .js to relative import/export paths across source and tests (including barrels like utils/index.js and data-sources/index.js). No controller or business logic changes.

ESLint now applies n/file-extension-in-import to those three asset-related package globs (plus the newly listed network-enablement-controller entry). Messenger action type scripts in both assets package.json files gain --esm so generated types use .js specifiers (e.g. AssetsController.js).

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