LibCore: Add function for searching a file in $PATH

This extracts the logic of searching for a file in $PATH from
System::exec to a separate function.

Userland/Libraries/LibCore/System.h

@@ -160,6 +160,7 @@ ErrorOr<Array<int, 2>> pipe2(int flags);
 #ifndef AK_OS_ANDROID
 ErrorOr<void> adjtime(const struct timeval* delta, struct timeval* old_delta);
 #endif
+ErrorOr<String> find_file_in_path(StringView filename);
 enum class SearchInPath {
     No,
     Yes,