Add environment stancode, put all Stan code in that environment

# Summary

For the Stan manual:

- Create a new environment definition, ``stancode`` that inherits from the ``Verbatim`` environement
- Put all Stan code chunks currently in ``Verbatim`` environment into ``stancode`` environments
- Within Stan code, make the code closer to valid code by commenting out lines beginning with ``...``

These changes will make it easier to change the formatting of Stan code chunks in the future. Currently each chunk has additional formatting like being placed in a quote environment and the fontsize changed. With a new environment the formatting of all chunks can be changed by changing the definition of the environment.  This would make it easier to make changes to the appearance of code, like in issue #1132 .

# Verify

The manual should compile without errors. The visual appearance of the manual should remain the same (except for the commented out lines with ``...``).