Add AffectedFile.Extension() in presubmit_support

Add convenience method AffectedFile.Extension() for extracting file
extension in a standard way. Add a test for this new functionality.
Add a similar test for AffectedFile.UnixLocalPath().

Bug: None
Change-Id: If591e751fb2e4fb5355ad0b6f93f310667849d68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6273341
Commit-Queue: Anton Bershanskyi <bershanskyi@gmail.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

presubmit_support.py

@@ -1041,10 +1041,10 @@ class AffectedFile(object):
         self._path = path
         self._action = action
         self._local_root = repository_root
-        self._is_directory = None
+        self._diff_cache = diff_cache
         self._cached_changed_contents = None
         self._cached_new_contents = None
-        self._diff_cache = diff_cache
+        self._extension = None
         self._is_testable_file = None
         logging.debug('%s(%s)', self.__class__.__name__, self._path)
 
@@ -1161,6 +1161,20 @@ class AffectedFile(object):
         self._cached_changed_contents = result
         return self._cached_changed_contents[:]
 
+    def Extension(self):
+        """Returns the file extension as a string.
+
+        File extension is the portion of file name after the last dot, including the dot.
+        If file name has no dot (like 'OWNERS'), then returns an empty string ''.
+        If file has multiple extensions (like 'archive.tar.gz'), then the last
+        extension is returned (like '.gz')
+        This method is equivalent to NodeJS node:path method path.extname()
+        and Python3 pathlib PurePath.suffix() and os.path.splitext()
+        """
+        if self._extension is None:
+            self._extension = os.path.splitext(self._path)[1]
+        return self._extension
+
     def __str__(self):
         return self.LocalPath()