Migrate to eslint flat config

[acelaya]

Eslint has a new recommended configuration system (AKA flat config) which is supported by v8, and will be the default in v9.

In order to migrate to it, these are the requirements.

• Replace the eslintrc file with a new eslint.config.js file. Migration guide: https://eslint.org/docs/latest/use/configure/migration-guide
• Replace @typescript-eslint/eslint-plugin and @typescript-eslint/parser with the new typescript-eslint package: Migration docs can be found here https://typescript-eslint.io/blog/announcing-typescript-eslint-v7/
• Wait until eslint-plugin-jsx-a11y has been migrated to support flat config: https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/pull/891
• Migrate our own eslint-config-hypothesis to support flat config: https://github.com/hypothesis/frontend-toolkit/issues/48

At the time of writing this, eslint only supports js extension for the flat config file. However, mjs and cjs support has been implemented and added to eslint 9 alpha versions.

This means the file needs to be written in CJS format as the package is not defined with type module. We might want to change that at some point.

[acelaya]

Once all pieces involved support flat config, the new config file should look something on these lines:

const tseslint = require('typescript-eslint');
const hypothesis = require('eslint-config-hypothesis');
const jsxA11Y = require('eslint-plugin-jsx-a11y');

module.exports = tseslint.config(hypothesis, jsxA11Y, {
  plugins: {
    '@typescript-eslint': tseslint.plugin,
  },
  languageOptions: {
    parser: tseslint.parser,
  },
  rules: {
    // All rules as defined currently in .eslintrc file
  },
});