Add formatting support for CSS

[ryanfowler]

No description provided.

[Copilot]

Pull request overview

This PR adds CSS formatting support to the fetch tool, enabling automatic formatting and syntax highlighting for CSS responses and CSS embedded within HTML <style> tags.

Changes:

• Implemented a CSS tokenizer and formatter with syntax highlighting and pretty-printing
• Integrated CSS formatting into HTML formatter for embedded styles
• Added CSS content type detection and formatting in the HTTP response handler

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/format/css.go	New CSS tokenizer and formatter implementation with support for selectors, at-rules, declarations, and nested structures
internal/format/css_test.go	Comprehensive test suite covering CSS tokenization, formatting, and edge cases
internal/format/html.go	Modified to format CSS content within <style> tags using the new CSS formatter
internal/format/html_test.go	Added tests for HTML with embedded CSS formatting
internal/fetch/fetch.go	Added TypeCSS constant and integrated CSS formatting into response handling
docs/USAGE.md	Updated documentation to list CSS as a supported format
README.md	Updated feature list to include CSS formatting

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