Add --default-extension argument to force file format

[mre]

Summary

Fixes #1665 by implementing a --default-extension option that allows users to specify a default file extension for files without extensions.

Problem

Users needed a way to manually specify the file format for files without clear extensions. This is common with:

• README files without extensions
• Configuration files
• Scripts without standard extensions
• Files with ambiguous extensions

Solution

Added --default-extension CLI option that:

• Accepts any valid file extension (e.g., md, html, txt)
• Uses the extension to determine the file type for processing
• Gracefully falls back to plaintext for unknown extensions
• Follows the same pattern as the existing --extensions option

Examples

# Treat README as markdown
lychee --default-extension md README

# Treat index as HTML
lychee --default-extension html index  

# Treat CHANGELOG as plaintext
lychee --default-extension txt CHANGELOG

# Works with any supported extension
lychee --default-extension htm file_without_ext
lychee --default-extension markdown docs

Implementation Details

• CLI Option: Added --default-extension argument to Config struct
• File Type Processing: Enhanced FileType::from_extension() to be public
• Input Processing: Updated inputs() method to convert extension to FileType hint
• Error Handling: Invalid extensions gracefully fall back to default plaintext behavior
• Testing: Comprehensive unit and integration tests

Design Decision: Extensions vs Types

Initially considered using abstract types (--default-type markdown), but switched to extensions (--default-extension md) because:

• More intuitive: Users work with familiar file extensions
• More flexible: Easy to support any extension lychee recognizes
• Consistent: Matches the existing --extensions option pattern
• Extensible: Could easily support multiple extensions in the future

Test Plan

• Unit tests for FileType::from_extension() with valid/invalid extensions
• Integration tests for CLI option with different file types
• Error handling test for unknown extensions
• Manual testing with various file scenarios
• All existing tests still pass

Verification

Before:
Files without extensions were always treated as plaintext, potentially missing links in markdown/HTML content.

After:

$ lychee --default-extension md README
# Now extracts markdown links from README file

$ lychee --default-extension html index  
# Now extracts HTML links from index file

The feature integrates seamlessly with existing functionality while providing users the control they need over file type detection.

[thomas-zahner]

I'm really confused by this sentence 😅

Suggested change

	Default file extension to treat files without extensions as having.
	This is the default file extension that is applied to files without an extension.

[mre]

Probably just me editing the wording over and over again until I broke the grammar.

[thomas-zahner]

@mre I noticed now that you merged your version. Did you forget about changing it? I still think this original version is quite confusing 😄

[mre]

Oh, that was by mistake. I thought I updated the text but apparently not.