Migrate tests to Playwright

[JayPanoz]

This primarily migrate tests from Backstop to Playwright in order to upgrade to Node 24 LTS. It also takes care of modules (esm).

[Copilot]

Pull request overview

This PR migrates the repo’s visual regression testing from BackstopJS to Playwright to enable upgrading the tooling to Node 24, while also converting several Node scripts/configs to ESM.

Changes:

• Replace BackstopJS-based visual regression with Playwright tests, snapshots, config, and CI workflows.
• Switch the project to ESM ("type": "module") and update Node scripts/config (server, PostCSS config, vars-doc generator) accordingly.
• Update documentation and ignore patterns to reflect the new Playwright workflow and artifacts.

Reviewed changes

Copilot reviewed 18 out of 21 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
tests/visual.spec.js	Adds Playwright-based screenshot tests covering the existing visual scenarios.
tests/ligatures.html	Adds a new ligatures test page used by the Playwright suite.
tests/base-rtl.html	Adjusts RTL test page to set a column-count user setting inline.
tests/arabic-ligatures.html	Removes the old Backstop ligatures scenario page.
tests/a11y-font.html	Removes the old Backstop a11y font scenario page.
playwright.config.js	Introduces Playwright configuration (snapshots, reporters, webServer).
package.json	Sets ESM mode and swaps BackstopJS for @playwright/test; updates test scripts.
package-lock.json	Updates dependency lockfile to remove BackstopJS/Puppeteer and add Playwright.
backstop.json	Removes BackstopJS configuration (no longer used).
.github/workflows/playwright.yml	Adds CI workflow to run Playwright visual regression on PRs/pushes.
.github/workflows/update-snapshots.yml	Adds a workflow to update and auto-commit Playwright snapshots.
.github/workflows/node.js.yml	Updates CI Node version to 24.x.
README.md	Updates developer instructions from Backstop to Playwright (install + update snapshots).
docs/CSS22-npm.md	Updates docs to describe Playwright testing and updated scripts.
docs/ReadiumCSS_docs/OEBPS/Text/Section-022.xhtml	Updates EPUB docs section for Playwright and ESM-oriented PostCSS snippets.
server.js	Converts the dev server to ESM for Node 24 compatibility.
scripts/generate-vars-doc.js	Converts vars-doc generator script to ESM.
css/postcss.config.js	Converts PostCSS config to ESM and updates imports/messages.
.nvmrc	Updates local Node version to v24.
.gitignore	Adjusts ignored test artifacts for the new Playwright workflow.

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[Copilot]

Pull request overview

Copilot reviewed 18 out of 21 changed files in this pull request and generated 3 comments.

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