Bug 1813299 - Filter out symlinks for webkitdirectory. r=smaug, a=dmeehan

This is effectively a backout of the following patches from Bug 1274959 except
we add comments and test coverage:
- Part 1 which made the DirectoryListingTask include symlinks in the results
  as exposed by Directory.getFilesAndDirectories.
  https://hg.mozilla.org/mozilla-central/rev/3f1e16d3bfb2
- Part 3 which made GetFilesHelper include symlinks in the results.
  https://hg.mozilla.org/mozilla-central/rev/e28c3a696524

Test coverage for getFilesAndDirectories is provided by
dom/filesystem/tests/test_basic.html by way of changes to its included file
dom/filesystem/tests/filesystem_commons.js and changes to the createTreeFile
helper in dom/filesystem/tests/script_fileList.js.

Test coverage for GetFilesHelper is provided by
dom/filesystem/tests/test_webkitdirectory.html and changes to the
createTestFile helper in dom/filesystem/tests/script_fileList.js.

Commenting out either of the `isLink` test in the relevant C++ code will cause
the given tests to fail on non-windows platforms.

Differential Revision: https://phabricator.services.mozilla.com/D178894