Replace addlicense Go dependency with a Python script

[DanielRyanSmith]

Background & Motivation

The wpt-gen repository currently uses the addlicense tool (written in Go) to enforce and apply Apache 2.0 license headers to files. This requires downloading and compiling a Go binary during the build/lint phase, adding an unnecessary toolchain dependency to what is otherwise a Python-centric project. By replacing addlicense with a custom Python script, we can simplify our development environment and make the project more portable. This aligns with the approach recently adopted in ChromeStatus (see chromium-dashboard/pull/6271).

Proposed Changes

1. Create License Checker Script:
   • Add a new Python script at scripts/check_license.py.
   • The script should recursively scan project files, respecting standard ignored directories (e.g., .venv/, .git/, dist/, .pytest_cache/, and others currently defined in ADDLICENSE_IGNORE).
   • The script must verify that supported source files (such as .py, .html, .js) contain the appropriate "Google LLC" Apache 2.0 license header.
   • Implement a --fix argument that automatically prepends the correct license header to files that are missing it.
2. Add Tests:
   • Create a corresponding scripts/check_license_test.py (or add it to the tests/ directory) to verify the license checker and fixer logic works as expected.
3. Update Makefile:
   • Remove the ADDLICENSE (go run github.com/google/addlicense@latest) and ADDLICENSE_IGNORE variables.
   • Update the license-check target to execute python scripts/check_license.py.
   • Update the license-fix target to execute python scripts/check_license.py --fix.
4. Cleanup:
   • Ensure all previous references to the Go toolchain and addlicense are removed from the project's documentation or configuration files.

Acceptance Criteria

• A Python script successfully checks for missing license headers across the repository.
• The script can automatically fix missing license headers using a --fix flag.
• Tests are added and passing to verify the behavior of the license checking script.
• The Makefile relies entirely on the new Python script instead of the Go addlicense dependency.
• make lint and make lint-fix continue to verify and fix licenses successfully.
• All references to addlicense and its Go dependency are removed from the project.

_{This issue description was drafted by Gemini}