isRelativePath function
Whether path is a relative path (no leading / and no drive letter).
Empty or whitespace-only input is treated as relative.
Example:
isRelativePath('docs/readme.md'); // true
isRelativePath('/etc/hosts'); // false
isRelativePath(r'C:\Windows'); // false
Implementation
bool isRelativePath(String path) {
final String trimmed = path.trim();
if (trimmed.isEmpty) return true;
if (trimmed.startsWith('/')) return false;
return trimmed.length < 2 || trimmed[1] != ':';
}