USWDS - JS: Add ESLint rules that can detect CommonJS/ESM mistakes #6849

Summary

eslint.config.mjs currently sets languageOptions.sourceType: "module"
for every file in packages/, and injects Node's require/module
globals for those same files -- so ESLint currently cannot distinguish a
CommonJS file from an ESM file, and cannot catch the most common mistake
that will come up while converting files from one to the other (see the
parent issue #6845 for full context). This issue adds the rules that make that
distinction possible, ahead of any actual source conversion.

The specific mistake this needs to catch

When a CommonJS module does module.exports = { prefix: "usa" }, converting
its importers to import { prefix } from "./config.js" looks correct but
throws at runtime: SyntaxError: Named export 'prefix' not found. The
correct, universally-safe form is
import config from "./config.js"; const { prefix } = config; (default
import, then destructure). This is exactly the class of error eslint-plugin-import-x's
import/named rule is designed to catch.

Scope

Restructure eslint.config.mjs into (at least) these scoped blocks, adding
rules only where they apply:

• ESM source block (files: ["packages/**/*.js", "packages/**/*.mjs"],
  excluding packages/usa-icon/src/usa-icons.config.js -- see note
  below): add
  - import/named: "error"
  - import/no-cycle: "error" (there are currently zero import cycles
  in packages/ source; this rule keeps it that way, which matters
  because a later part of this migration relies on the dependency
  graph staying acyclic)
  - import/extensions: ["warn", "ignorePackages", { js: "always", mjs: "always" }]
  -- land this as a warning, not an error, since it will fire on
  every extensionless relative require() call currently in the
  codebase (there are many). A shrinking warning count in CI output
  over the course of this migration is a useful progress signal for
  reviewers. It gets flipped to "error" in the final cleanup issue of this migration (USWDS - JS: Migration cleanup -- delete the Storybook CJS-to-ESM transform plugin, tighten lint rules, fix stale docs #6869), once no CommonJS source remains.
  - import/no-commonjs: leave this off in this issue. It gets
  turned on incrementally, one directory at a time, by each later
  conversion issue in this migration, as that directory's files
  glob -- turning it on globally now would just fail on every
  unconverted file and add no signal.
• Node-tooling block (gulpfile.js, tasks/**/*.js (but not
  tasks/**/*.mjs, which are already ESM), .storybook/**/*.js,
  svgo.config.js, webpack.twig.config.js,
  packages/usa-icon/src/usa-icons.config.js): explicitly set
  languageOptions.sourceType: "commonjs" and keep Node globals. This
  block should have import/no-commonjs explicitly disabled (these
  files are meant to stay CommonJS).
• Note: packages/usa-icon/src/usa-icons.config.js lives inside
  packages/ but is a Node-side build config
  (tasks/svg-sprite.js does require("../packages/usa-icon/src/usa-icons.config")),
  not browser-facing component source. It must be excluded from the
  "ESM source" block above and included in the "Node-tooling" block, or
  it will be mistakenly targeted by later conversion work.
• npm run lint:js passes with only the expected import/extensions
  warnings (no new errors).

Verification

npm run lint:js

Confirm the warning count roughly matches the number of extensionless
relative require()/import specifiers currently in packages/ (order of
magnitude: over 100).

Depends on

Should land after "Tooling hygiene ahead of the ESM migration" (#6846; which
widens lint:js's scope to the whole repo -- this issue's Node-tooling
block needs that widened scope to have any effect).