feat(cli): add --format option for html output

[Divyapahuja31]

Description

This PR introduces a new --format option to the @shikijs/cli, enabling users to output syntax-highlighted code as HTML, in addition to the existing ANSI terminal output.

Changes:

• Added --format <format> flag (supports ansi | html, default: ansi).
• Integrated codeToHtml for HTML output generation.
• Added comprehensive test case for the new option.
• Updated CLI documentation.

Why this is valuable:

1. Scripting & Automation: Users can now easily pipe highlighted HTML into files for static sites, documentation, or reports without writing custom Node.js scripts.
   • Example: npx shiki-cli file.js --format html > snippet.html
2. Debugging: Provides a quick way for maintainers and users to inspect the exact HTML output Shiki generates for a specific code/theme combination directly from the terminal.
3. Feature Parity: Aligns Shiki CLI with other standard highlighting tools (like pygments) that support multiple output formats.

Implementation Details:

• The implementation leverages the existing codeToHtml function from the core package, ensuring consistent output with the main library.
• The default behavior remains unchanged (ANSI output), ensuring no breaking changes for existing workflows.

Verification:

• Added a new test in packages/cli/test/cli.test.ts to verify that --format html correctly produces an HTML string with the expected class names.
• Ran pnpm test packages/cli -> All tests passed.
• Ran pnpm build -> Build successful.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.20%. Comparing base (61c87e8) to head (91bd284).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   95.20%   95.20%           
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  Files          92       92           
  Lines        7922     7922           
  Branches     1689     1689           
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  Hits         7542     7542           
  Misses        374      374           
  Partials        6        6           

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