chore: Upgrade to eslint9

[bingenito]

• Introduced ESLint9 configuration file with rules and license header enforcement.
• Added license header to missing files via lint:fix
• Configured ESLint to ignore specific directories and files, including build artifacts and configuration files.

[Copilot]

Pull Request Overview

This PR upgrades the project to ESLint9, adds automatic license header enforcement to files missing them, and updates the linter configuration and documentation.

• Introduces a new ESLint9 configuration file with an enforced license header rule.
• Adds license header comments to various configuration, test, and source files.
• Updates ignore patterns and README instructions for linting.

Reviewed Changes

Copilot reviewed 10 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/desktopjs/vite.config.ts	Added license header at the top.
packages/desktopjs/tests/setup.ts	Added license header to the Jest setup file.
packages/desktopjs-openfin/vite.config.ts	Added license header at the top.
packages/desktopjs-openfin/tests/openfin.spec.ts	Updated license header and adjusted event listener type annotations.
packages/desktopjs-openfin/src/openfin.ts	Replaced ternary with explicit if/else in getState for clarity.
packages/desktopjs-electron/vite.config.ts	Added license header at the top.
packages/desktopjs-electron/tests/setup.ts	Added license header to the Jest setup file.
eslint.config.mjs	New ESLint9 configuration including license header enforcement.
README.md	Added linting instructions and commands.

Files not reviewed (3)

• .eslintrc.json: Language not supported
• package.json: Language not supported
• tsconfig.test.json: Language not supported