Feature: Update renderer to take in a user-created Context type

[mix3d]

Since the render contexts are currently hidden from the export, there isn't an easy way to extend context classes at the consumer level (without modifying vexflow source directly).

Assuming a new Context class implements / inherits the needed base Context functions (see @types/vexflows IRenderContext class for example), there shouldn't be any reason why the Renderer couldn't handle being passed in a custom Context class to instantiate itself with.

Another way to solve this issue, (that might allow for user error, however), would be to expose the Backend classes somewhere at the root level, such that a developer could extend/override those classes at runtime.

Either of these could be a solution to #722, letting a dev like myself fix in my own code, making it easier to introduce non-compliant but implementation specific features without having to fork the root project.