Add Custom File Type Handler API

[bajrangCoder]

This PR introduces a file type handler system that allows plugins to register custom handlers for specific file types. This enables plugins to provide specialized viewers or editors for different file formats (e.g., PDF viewers, image editors, etc.).

Key Features

• Plugins can register handlers for specific file extensions
• "Text Editor" always available as fallback option

Example Usage

acode.registerFileHandler({
  id: 'my-pdf-viewer-id',
  extensions: ['pdf'],
  async handleFile({ name, uri, fs, options }) {
    // Handler implementation
  }
});

[Copilot]

Pull Request Overview

This PR introduces a file type handler system that allows plugins to register custom handlers for specific file types, enabling specialized viewers or editors to be used for different file formats.

• Added new CSS file references in www/index.html
• Integrated file type handling into openFile.js with a new fileTypeHandler module
• Extended the Acode class to expose handler registration and unregistration methods

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
www/index.html	Added new CSS file references and removed outdated ones
src/lib/openFile.js	Integrated custom file handler lookup and invocation
src/lib/fileTypeHandler.js	Introduced a registry class for managing custom file handlers
src/lib/acode.js	Exposed API methods for registering and unregistering handlers