Make optional jQuery require bundler-friendly (#4184)

[BigBalli]

Fixes #4184.

Problem

The CommonJS branch of the UMD wrapper does:

var _ = require('underscore'), $;
try { $ = require('jquery'); } catch (e) {}

The try/catch is correct at runtime in Node.js, but Webpack, Rollup,
esbuild, packd and other bundlers statically analyze the literal
require('jquery') call before the try/catch ever runs. Users who
don't actually want jQuery still see their bundles fail with
Cannot find module 'jquery'.

Fix

Indirect the optional require through a variable so the bundler's
static analysis does not follow it:

var nodeRequire = typeof require === 'function' && require;
try { if (nodeRequire) $ = nodeRequire('jquery'); } catch (e) {}

This is the same trick used by ws and other libraries with optional
native deps. Runtime behavior is unchanged: the require still happens
in Node, the try/catch still swallows ENOENT, and Backbone.$ is
still populated when jQuery is installed.

Also declare jquery as an optional peer dependency in package.json
so npm/yarn/pnpm document the relationship cleanly without forcing the
install:

"peerDependencies": { "jquery": ">=1.11.0" },
"peerDependenciesMeta": { "jquery": { "optional": true } }

Test

• npm run lint passes.
• Smoke-tested require('./backbone.js') in a fresh Node process with
  no jQuery installed: loads cleanly, Backbone.VERSION === '1.6.1',
  Backbone.$ === undefined as expected.

[jgonggrijp]

Thank you for the awesome work in this pull request and the previous four!

I made a superficial glance over all five PRs and they look very good to me. I will be reviewing them in more detail (and almost certainly also merging them) over the next two weeks or so. Do you expect any merge conflicts between them, and if so, which order would you recommend?