Add maintenance plan

[bregydoc]

This pull request adds a PLAN_DE_MANTENIMIENTO.md file containing an English maintenance plan for PGOJs. Key tasks include:

• Switch to Go modules for dependency management and update to the latest Go, gopherjs, and p5.js versions (or explore Go’s WebAssembly support).
• Separate library code from examples, add a go.mod and proper packages, and tag releases with semantic versioning.
• Provide build instructions for compiling to JavaScript via gopherjs or WebAssembly and optionally publish pre-built bundles.
• Improve documentation by expanding the README with API details, supported features, limitations, and step-by-step tutorials; update and add more examples.
• Add unit tests and configure GitHub Actions to run go test, go vet, and linting (e.g., golangci-lint) on multiple Go versions.
• Engage the community by triaging issues and PRs, adding a CONTRIBUTING guide, and discussing future features such as WebAssembly support.
• Define a roadmap for porting more of the p5.js API, enhancing performance, and maintaining compatibility with modern browsers.

Implementing this plan will modernize the project and encourage community collaboration.