Handle null status in isDdevRunning and use separate args for PHPStan options

[Copilot]

Addresses review feedback from PR #56 to improve error handling and command argument consistency.

Error handling in isDdevRunning:

• Added explicit checks for result.error and result.status === null to distinguish between "ddev not running" vs "ddev command not found/failed" scenarios
• Display appropriate error messages for each failure mode

PHPStan command building:

• Changed --level=${value} and --exclude=${path} to separate arguments: '--level', value and '--exclude', path
• Properly handles special characters and spaces in paths with spawnSync's array-based execution

// Before
command.push(`--level=${this.config.level}`);
command.push(`--exclude=${excludePath}`);

// After
command.push('--level', String(this.config.level));
command.push('--exclude', excludePath);

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