section headers and footers do not abide by comment_prefix setting

Section headers and footers do not currently abide by the comment_prefix setting, and as such, can break region folding if used for the section headers/footers as seen below.

example config:

comment_prefix = ""
import_heading_stdlib = "#region Standard Library"
import_footer_stdlib = "#endregion Standard Library"

expected output:

#region Standard Library

import sys

#endregion Standard Library

actual output:

# #region Standard Library

import sys

# #endregion Standard Library

Start by tracing how comment_prefix is applied to import_heading_stdlib and import_footer_stdlib, then add a regression test for the configuration and expected output shown here. Done means section headers and footers are emitted without an extra comment prefix when comment_prefix is empty, while the existing behavior remains covered.