Fix Windows path resolution for drive-letter paths

[krekas]

Fixes #22 - Windows path resolution bug when running filacheck on a Windows Dev drive (e.g. D:\example).

The absolute path check only looked for Unix-style / prefix, so Windows drive-letter paths like D:\example/app/Filament were treated as relative and prepended with getcwd() again, producing an invalid duplicated path.

Before

// Only detects Unix absolute paths
if (! str_starts_with($path, '/')) {
    $path = getcwd().'/'.$path;
}

// On Windows with D:\example:
// getcwd() returns "D:\example"
// $path = "D:\example/app/Filament" — does NOT start with "/"
// Result: "D:\example/D:\example/app/Filament" — BROKEN

After

function isAbsolutePath(string $path): bool
{
    return str_starts_with($path, '/') || preg_match('/^[A-Za-z]:[\\\\\/]/', $path) === 1;
}

if (! isAbsolutePath($path)) {
    $path = getcwd().DIRECTORY_SEPARATOR.$path;
}

// On Windows with D:\example:
// getcwd() returns "D:\example"
// $path = "D:\example\app\Filament" — isAbsolutePath() returns true
// Result: "D:\example\app\Filament" — CORRECT

Changes

• Added isAbsolutePath() helper that detects both Unix (/...) and Windows (C:\..., C:/...) absolute paths
• Replaced str_starts_with($path, '/') check with isAbsolutePath($path)
• Replaced hardcoded / separators with DIRECTORY_SEPARATOR when joining paths with getcwd()