Provide a `debug` flag to any snippet

In order to move forward with CV-19, we need to provide a way to enable a debugging flag to any kind of snippet we export.

Any snippet filter could be prefixed with a call to the snippet debug which enables debugging per each snippet/function we use.

The debug flag is setup in a way that each function could have more details in console, keeping the filter identical except for the optional debug prefix snippet.

example.com#$#log OK
VS
example.com#$#debug; log OK

What to change

• Add the debug snippet in the snippet library
• Add debug support to the log snippet, prefixing the log output with DEBUG
• Add test

Hints for tester

• Create a log snippet filter
• Test it work as expect
• Modify the filter to add debug
• Test it shows the debug output