Update babel monorepo to v8

[rubensworks]

Supersedes #188, which was green everywhere except lint.

Why #188 failed

Babel 8 packages are ESM-only, and ESLint 7 loads parsers with require():

/home/runner/work/jQuery-Widget.js/jQuery-Widget.js/node_modules/@babel/eslint-parser/lib/index.js:1
(function (exports, require, module, __filename, __dirname) { import { types, version } from '@babel/core';
                                                              ^^^^^^
SyntaxError: Cannot use import statement outside a module

@babel/eslint-parser@8 also declares eslint@^9.0.0 || ^10.0.0 as a peer dependency, so the babel bump cannot land without moving off ESLint 7.

Changes

• @babel/core and @babel/preset-env to ^8.0.0, @babel/eslint-parser to ^8.0.0 (same as Update babel monorepo to v8 #188).
• eslint to ^9.0.0, plus globals as a dev dependency.
• .eslintrc replaced by an eslint.config.mjs flat config. All rules are carried over unchanged, except:
  • valid-jsdoc was dropped, as ESLint 9 removed the rule. It was already disabled (0).
  • env: { es6, node, browser } became the equivalent languageOptions.globals spreads of the globals package.
  • comma-dangle is now spelled out per syntax kind. The always-multiline shorthand used to leave function calls unchecked because the config targeted an ES version below 2017; with ecmaVersion: 'latest' the shorthand would newly flag call sites, so functions: 'ignore' keeps the previous behaviour.
  • no-unused-vars gets caughtErrors: 'none', as ESLint 9 changed that default from none to all.

No source files needed changes.

Verification

Ran the full CI matrix locally on Node 22: yarn run lint, yarn run dev-ci, yarn run dev-prod-ci and yarn run build all succeed. Also confirmed the new lint config still reports errors (deliberately introduced no-unused-vars, semi-spacing and no-extra-semi violations were caught), so linting is not silently passing.

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Generated by Claude Code

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