Evergreen: the modernization release (ES2022, native fetch, Vitest, Yarn 4)

Drops the dead IE6/IE7 code paths, modernizes prototype method syntax to
ES6 shorthand, ships a native fetch() fallback for Backbone.ajax,
and moves the project off npm onto Yarn 4.

Added

• Backbone.ajax falls back to native fetch() when Backbone.$
  is absent. Previously the library was unusable without jQuery (or a
  hand-rolled Backbone.ajax override). The fallback maps the subset
  of jQuery options that Backbone.sync emits — type, url, data,
  contentType, dataType, headers, beforeSend, success,
  error — onto fetch() + AbortController, and returns a thenable
  carrying .abort() so the var xhr = model.fetch(); xhr.abort();
  consumer pattern keeps working. The jQuery path is preserved verbatim
  whenever Backbone.$ is present.

Removed (breaking for IE6/IE7)

• The hidden iframe trick that simulated hashchange for IE7, plus
  the four call sites in History that special-cased this.iframe.
• The setInterval polling fallback for browsers without
  onhashchange, and the orphan History#interval knob that only
  existed to feed it.
• The attachEvent / detachEvent shims around
  addEventListener / removeEventListener.
• The document.documentMode > 7 gate that guarded against IE7's
  broken onhashchange.
• Zepto and Ender from the browser-global $ resolution chain in the
  UMD factory; only jQuery (or $) remains.
• The IE6 hash/search and Firefox location.hash-decoding workaround
  comments. History#getHash also loses its unused window argument.
• The attachEvent / detachEvent ESLint globals.

Changed

• Every prototype method literal on Model, Collection, View,
  Router, and History now uses ES6 method shorthand:
  methodName(args) { ... } instead of
  methodName: function(args) { ... }. Pure syntactic, no behavior
  change. The function-style constructor wrappers
  (var Model = Backbone.Model = function(...) {...}) are preserved
  on purpose: converting them to class would break the documented
  Backbone.Model.apply(this, arguments) pattern in user-provided
  constructor: overrides (exercised by test jashkenas#2612), since super()
  must come first in a derived class constructor.
• Backbone.VERSION, the source header, and package.json are all
  aligned at 1.7.0.

Tooling

• Package manager: npm → Yarn 4.14, pinned via Corepack and the
  packageManager field. node-modules linker for full compatibility
  with Karma/Rollup/Docco; no Zero-Install (cache stays out of git).
  package-lock.json is replaced by yarn.lock.
• CI Node 14 → 20, required by Yarn 4 (≥ 18.12). npm ci becomes
  yarn install --immutable.
• dependenciesMeta.puppeteer.built is set explicitly so Yarn 4's
  default postinstall block doesn't strand the Chromium download that
  karma.conf.js depends on.