Added support for Shapely offset using a plugin implementation.
There are a few things worth discussing about it, mainly concerning the different arguments that compas and shapely functions require. If we want to keep the existing compas functions, for backward compatibility, then we could add in the now-plugin functions and the in-class calls a few lines indication the different args a user can input depending on the backend they are using.
Additionally, I rephrased the original shapely corner style options from 'miter, bevel, round' to 'sharp, chamfer, sharp' to match the ones in Rhino. Seeing as most users of compas will be more familiar with Rhino's terminology and the potential addition of rhino plugins for them, I thought it's more fitting!
Let me know your thoughts :)

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)