Modernize JavaScript build toolchain

[tagliala]

Replace Rollup with Rolldown v1 for bundling (10-30x faster builds). The new config lives in rolldown.config.mjs and the build script is updated accordingly.

Replace ESLint + neostandard with Oxlint v1 for linting (50-100x faster). The new config lives in .oxlintrc.json and is run via pnpm lint. Use args: none on no-unused-vars to skip unused function parameters, which serve as API documentation for callback signatures. Rename .github/workflows/eslint.yml to lint.yml accordingly.

These choices are informed by ongoing upstream discussions about how best to align the neostandard and Rollup ecosystems with modern JavaScript tooling (neostandard/neostandard#350, rollup/plugins#2010).

Drop Babel — transpilation is now handled by Rolldown's built-in Oxc transforms. The minimum browser targets are unchanged (Chrome 60+, Firefox 60+, iOS 12+, Safari 12+).

Remove 9 unused npm devDependencies (@babel/core, @babel/preset-env, @rollup/plugin-babel, @rollup/plugin-node-resolve, eslint, eslint-plugin-compat, neostandard, rollup, rollup-plugin-copy).

Clean up .gitignore (remove stale !.babelrc negation) and update Rakefile lint task to call the new pnpm lint script instead of pnpm eslint.

[Copilot]

Pull request overview

This PR modernizes the JavaScript toolchain by switching bundling from Rollup+Babel to Rolldown (Oxc transforms) and switching linting from ESLint+neostandard to Oxlint, with corresponding script/workflow/task updates. It also regenerates the distributed JS artifacts to reflect the new bundler output and removes now-unused configuration/dependencies.

Changes:

• Replace Rollup+Babel with Rolldown v1 for builds and add rolldown.config.mjs.
• Replace ESLint+neostandard with Oxlint v1, add .oxlintrc.json, and update CI/Rake tasks to run pnpm lint.
• Remove Babel/Rollup/ESLint-related devDependencies/config and refresh generated dist/ + vendor/ JS outputs.

Reviewed changes

Copilot reviewed 9 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
vendor/assets/javascripts/rails.validations.js	Regenerated vendored UMD bundle output from the new bundler.
rollup.config.mjs	Removed Rollup configuration as bundling migrates to Rolldown.
rolldown.config.mjs	Added Rolldown configuration and post-processing/copy steps for dist + vendor outputs.
Rakefile	Updates JS lint task to run the new pnpm lint script.
package.json	Updates build/lint scripts and removes obsolete Rollup/Babel/ESLint devDependencies.
eslint.config.mjs	Removed ESLint+neostandard flat config.
dist/client-side-validations.js	Regenerated distributed UMD bundle output from the new bundler.
dist/client-side-validations.esm.js	Regenerated distributed ESM bundle output from the new bundler.
CHANGELOG.md	Adds/rewords entries to document the toolchain breaking changes.
.oxlintrc.json	Adds Oxlint configuration (including no-unused-vars args handling).
.gitignore	Removes stale !.babelrc negation now that Babel config is removed.
.github/workflows/lint.yml	Renames/updates lint workflow/job to run pnpm lint.
.babelrc	Removed Babel configuration (Babel no longer used).

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