refactor(path): use null-aware elements in join and absolute

Replace list-based argument validation with individual parameter
validation to avoid list allocation overhead. Use Dart 3.8 null-aware
elements (?expr) in list literals to build filtered lists directly
instead of using whereType<String>().

- Add _validateArgs() that operates on 16 String? arguments directly
- Add _throwArgError() helper for constructing error messages
- Update join() and absolute() to use null-aware elements                                                                                                  - Bump SDK constraint to ^3.8.0
                                                                                                                                                             Performance improvement: ~2x faster for absolute() operations.

Author: NazarenoCavazzon

pkgs/path/benchmark/benchmark.dart

@@ -58,6 +58,16 @@ void main(List<String> args) {
     }
 
     benchmark('absolute', context.absolute);
+    runBenchmark('${style.name}-join', 100000, () {
+      for (var file in files) {
+        context.join(file, 'subdir', 'file.txt');
+      }
+    });
+    runBenchmark('${style.name}-join-many', 100000, () {
+      for (var file in files) {
+        context.join(file, 'a', 'b', 'c', 'd', 'e', 'f');
+      }
+    });
     benchmark('basename', context.basename);
     benchmark('basenameWithoutExtension', context.basenameWithoutExtension);
     benchmark('dirname', context.dirname);