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 and enforces a standardized license header across multiple files. Key changes include:

• Adding a shared Apache License header to source and test files.
• Introducing a new ESLint9 configuration file with updated rules.
• Updating some test implementations for clarity and type safety.

Reviewed Changes

Copilot reviewed 9 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/desktopjs/vite.config.ts	Added license header.
packages/desktopjs/tests/setup.ts	Added license header.
packages/desktopjs-openfin/vite.config.ts	Added license header.
packages/desktopjs-openfin/tests/openfin.spec.ts	Updated tests and cleaned up comments while adding license header.
packages/desktopjs-openfin/src/openfin.ts	Replaced ternary in getState with explicit if/else for clarity.
packages/desktopjs-electron/vite.config.ts	Added license header.
packages/desktopjs-electron/tests/setup.ts	Added license header.
eslint.config.mjs	Introduced ESLint9 config with license header enforcement.

Files not reviewed (3)

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.03%. Comparing base (f8dc3b1) to head (29fec16).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #731      +/-   ##
==========================================
+ Coverage   73.98%   74.03%   +0.05%     
==========================================
  Files          16       16              
  Lines        1507     1510       +3     
  Branches      317      302      -15     
==========================================
+ Hits         1115     1118       +3     
- Misses        367      392      +25     
+ Partials       25        0      -25     

Flag	Coverage Δ
unittests	74.03% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
packages/desktopjs-openfin/src/openfin.ts	87.89% <100.00%> (+0.09%)	⬆️

... and 3 files with indirect coverage changes

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