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 by adding a new ESLint configuration file, enforcing license header compliance across files, and updating documentation for lint commands.

• Added standardized license headers to various source and test files.
• Updated ESLint configuration in eslint.config.mjs with new rules and plugins.
• Updated README.md to include linting instructions.

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 to the Vite configuration.
packages/desktopjs/tests/setup.ts	Added license header to the test setup file.
packages/desktopjs-openfin/vite.config.ts	Added license header to the OpenFin Vite configuration.
packages/desktopjs-openfin/tests/openfin.spec.ts	Made minor type adjustments and style improvements in test callbacks.
packages/desktopjs-openfin/src/openfin.ts	Refactored getState to use an explicit if/else for improved readability.
packages/desktopjs-electron/vite.config.ts	Added license header to the Electron Vite configuration.
packages/desktopjs-electron/tests/setup.ts	Added license header to the Electron test setup file.
eslint.config.mjs	Introduced updated ESLint configuration with license header enforcement.
README.md	Included new instructions for linting commands.

Files not reviewed (3)

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