Enable file:// hyperlinks in Rich Fixes #3786

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest [black](https://github.com/psf/black) with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

This PR fixes an issue where file:// hyperlinks were not working in Rich due to the underlying markdown-it-py implementation. The markdown-it-py library blocks file:// URLs for security reasons when rendering in browser environments, but this security restriction doesn't apply to Rich since it runs locally.

Problem

• file:// hyperlinks in Rich markup were not being rendered as clickable links
• This was caused by markdown-it-py's security policy that disables file:// URLs by default
• The security concern (preventing access to local files in web browsers) doesn't apply to Rich's local terminal use case

Solution

• Modified Rich's markdown processing to allow file:// URLs when rendering hyperlinks
• This change is safe for Rich's use case since the code runs locally anyway, not in a browser context
• Users can now create hyperlinks to local files using standard file:// URL syntax

Example Usage

from rich.console import Console
from rich.markdown import Markdown
console = Console()
markdown = Markdown("[Local File](file:///path/to/local/file.txt)")
console.print(markdown)