Add CLI tool for file conversion

[dereuromark]

Summary

Adds a command-line tool for converting djot files to HTML.

Usage

# Convert file to stdout
vendor/bin/djot document.djot

# Convert file to output file
vendor/bin/djot document.djot -o document.html

# Convert from stdin
echo '*hello*' | vendor/bin/djot

# With options
vendor/bin/djot document.djot --xhtml --safe --warnings

Options

Option	Description
-o, --output FILE	Write output to FILE instead of stdout
-x, --xhtml	Use XHTML-compatible output (self-closing tags)
-s, --safe	Enable safe mode (sanitize dangerous HTML)
-w, --warnings	Show parse warnings on stderr
--strict	Exit with error code 1 if warnings are generated
-h, --help	Show help message
-v, --version	Show version information

Test plan

• Tests pass
• Code style passes
• Manual testing of all options

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.72%. Comparing base (2946eb7) to head (f1e94d3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #54   +/-   ##
=========================================
  Coverage     93.72%   93.72%           
  Complexity     1928     1928           
=========================================
  Files            74       74           
  Lines          5177     5177           
=========================================
  Hits           4852     4852           
  Misses          325      325           

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