[2.x] fix(webpack): prevent Uncaught ReferenceError for async default exports

[rafaucau]

Changes proposed in this pull request:

• autoExportLoader.cjs: Fixed a regex bug that incorrectly captured the async keyword as an export name when using anonymous arrow functions (e.g., export default async () => {}). This prevents the Uncaught ReferenceError: async is not defined runtime error in extensions.
• babel.config.cjs: Dynamically set useESModules based on process.env.NODE_ENV === 'test'. This fixes the Jest test suite which was failing due to ESM imports being injected into the CommonJS test environment.
• Added tests for both named and anonymous async exports.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[imorland]

Thank you @rafaucau this is a much needed fix!