feat: Add htmlhint --init command (#1630)

* feat: Add `htmlhint --init` command

This creates a new config (if one doesn't already exist)

* make the --init command much more robust and follows standard CLI best practices

Updated behavior:
Exit code 0 (success):
✅ Config file created successfully
✅ Config file already exists (no action needed, but not an error)
Exit code 1 (failure):
❌ File creation failed due to write permissions, disk space, etc.
Why this is better:
Follows CLI conventions: Failed operations should exit with non-zero codes
Better for scripting: Scripts can check $? or $LASTEXITCODE to handle errors
More user-friendly: Existing file is treated as success, not failure
Proper error handling: Real write errors are properly reported
Updated tests:
✅ All tests pass
✅ Tests verify correct exit codes for each scenario
✅ Tests handle platform differences (Windows vs Unix)
This change makes the --init command much more robust and follows standard CLI best practices.

* Fix flaky test

Author: coliff