Extract stacktrace from eval

[platypii]

This PR adds stacktrace support to jscad.app

Previously this would have just given error.toString() but now it gives a full stacktrace.

Changes:

• Add formatStacktrace function which parses a stacktrace and extracts the lines which were inside the eval function.
• Remove noise from stacktrace introduced by eval
• Serialize stack specifically in postmessage because otherwise it gets lost in transit (necessary for firefox)