feat: Add code blocks transformation to Confluence code macro

[FrankLedo]

Summary

Implements proper rendering of markdown code blocks in Confluence by converting them to structured code macros with syntax highlighting support. This addresses issue #65 where code blocks were appearing as inline text instead of formatted code blocks.

Changes

• Added new rehype-replace-code-blocks plugin to transform <pre><code> elements to Confluence's code macro format
• Added confluence.rehype.codeBlocks configuration option (default: true) to enable/disable the feature for compatibility with older Confluence versions
• Comprehensive unit tests with 10 test cases covering various scenarios (empty blocks, multiple languages, special characters, etc.)
• Updated documentation in README.md with configuration details and examples
• Updated CHANGELOG.md under "unreleased" section
• Bumped package version to 2.3.0 (MINOR version per semantic versioning)

Implementation Details

The plugin:

• Converts markdown fenced code blocks to Confluence's <ac:structured-macro ac:name="code"> format
• Preserves language syntax highlighting information from code fence language identifiers
• Handles edge cases: empty blocks, blocks without language specification, special characters, multi-line code
• Is enabled by default but can be disabled via configuration for older Confluence instances

Test Plan

• All existing unit tests pass
• New unit tests added and passing (10 comprehensive tests)
• Linting checks pass
• TypeScript compilation successful
• Feature follows existing plugin patterns in the codebase

Closes #65

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[FrankLedo]

I have read the CLA Document and I hereby sign the CLA

[javierbrea]

Wow! Fantastic work, thank you so much for this contribution! 🙌

I really appreciate the thorough implementation and comprehensive test coverage. There are just two small details I'd like to suggest. I've left some detailed comments in the code review regarding the configuration structure and default values.

I'm happy to handle merging this into the release branch and exposing the new configuration option through the general configuration system. I won't ask you to do this part since the code for adding these options can be a bit tricky—and I don't think that's the best use of your time on a first contribution! 😊

[javierbrea]

Changing the base branch to feat/65/rehype-codeblocks to finish development there before adding it to the release branch.