feat: add Node ESM build: route the node import condition to a real Node bundle

[diegomura]

Problem

The root export map only defines require under the node condition:

".": {
  "node": { "require": "./js/pdfkit.js" },
  "default": "./js/pdfkit.browser.mjs"
}

When a Node ESM consumer does import PDFDocument from 'pdfkit', the nested node object has no matching key, so resolution falls through to "default" — and modern Node users silently get the browser build. On Node that means:

• Virtual fs instead of real fs: path-based APIs (doc.image('/path.png'), doc.font('/path.ttf'), doc.file('/path')) fail unless the file was pre-registered with registerFile.
• fflate's pure-JS zlibSync instead of native zlib.deflateSync for every content stream (a measurable perf regression on large documents).
• The custom browser Readable shim instead of Node streams.
• Standard fonts are not self-registering (the browser build expects the consumer to call registerStdFonts).

Nothing guarded this: tests/package-resolution.cjs only exercises require('pdfkit'). (Extra context: react-pdf is converging onto upstream pdfkit and is a pure-ESM consumer, so it currently lands on the browser build in Node.)

Change

Add a Node ESM bundle (js/pdfkit.node.mjs) and wire it into the export map:

".": {
  "node": {
    "import": "./js/pdfkit.node.mjs",
    "require": "./js/pdfkit.js"
  },
  "default": "./js/pdfkit.browser.mjs"
}

• The ESM bundle is a second output of the existing lib/document.node.js rollup entry, sharing its plugins, externals and babel config. #fs/#zlib/#stream are inlined via the node condition exactly as in the CJS bundle, with fs/zlib/stream external.
• lib/document.node.js now gets its require from createRequire(import.meta.url) so the lazy standard-font loaders work in both output formats (rollup shims import.meta.url in the CJS output; the AFM data modules are still only loaded when a standard font is actually used).
• Browsers/bundlers are unaffected: they don't match the node condition and keep resolving to js/pdfkit.browser.mjs via "default".

Tests

New tests/package-resolution.mjs, wired into npm run test:package:

• Asserts import.meta.resolve('pdfkit') points at js/pdfkit.node.mjs.
• Proves it's the Node build functionally: loads a font and an image by file path and ends the document cleanly (this fails on the browser build's virtual fs — verified).
• Proves standard fonts self-register: a default-Helvetica document with text ends cleanly.
• Proves laziness is kept: after using only Helvetica, no other standard-font data module is in the require cache.

tests/package-resolution.cjs previously used await import('pdfkit') to reach the browser bundle (the misresolution this PR fixes); it now asserts the dynamic import yields the Node ESM build and loads the browser ESM bundle directly by path to keep its coverage.

npm test (481 tests), npm run test:tools, npm run lint and npm run prettier all pass.

[diegomura]

After this I can start using pdfkit in react-pdf 😄 @blikblum I'd appreciate your review and re-publishing 🙏🏻 I think that will be big for this lib

[blikblum]

Done. BTW i'm surprised that it will work on ESM in node (due to some dependencies not being ESM friendly)