cosmos-explorer/canvas

Why?

This adds a mock module for canvas. Nteract has a ignored require and undeclared dependency on this module. cavnas is a server side node module and is not used in browser side code for nteract.

Installing it locally (npm install canvas) will resolve the problem, but it is a native module so it is flaky depending on the system, node version, processor arch, etc. This module provides a simpler, more robust solution.

Remove this workaround if this bug ever gets resolved