shadow-dom: Add test conversion script.

This script converts tests under the shadow-dom/ directory to separate HTML
files with appropriate metadata format. The output is produced within
the out/ directory by default to avoid conflicts with existing files, but
I intend that the files under the out/ directory will take over the existing
contents of the shadow-dom/ directory.

The script file will be removed after the conversion.
shadow-dom: Reorganize the test structure.

This change reorganizes the tests under the shadow-dom/ directory. In the new
format, each test has a dedicated, clean HTML with appropriate metadata.
The new format provides a clear directory structure based on the section
hierarchy of the specification. In addition, numerous issues with the existing
tests are addressed, such as unnecessary files, messy metadata definitions,
and so on.

The new files are generated by convert.py, which is included in the previous
commit.

There are some issues I could not address using a script; most notably
each test does not have a sensible name. These issues need to be addressed
manually, so I left these as future work.
shadow-dom: Remove convert.py.