fixQuotes should not rewrite the content of `filecontents(*)` environment

The Issue*

Please briefly describe the issue you come across.

• If there are double quotes in a filecontents(*) environment, and if fixQuotes is enabled, the quotes are replaced with [double backticks] and ''.
• The content of the filecontents(*) environment is not necessarily LaTeX code.

Reproduction Steps

Please list out the steps to reproduce your bug. Include relevant environmental variables or any other configuration.

1. Try to generate a file with the filecontents(*) environment containing ".
2. The double quotes get replaced.

fixQuotes should treat those environments as verbatim.