Add Mirror node

[skoriop]

This will add the Mirror node:

• [x] Define a new node that just outputs the vector data that has been input (to ensure nodes are set up properly).
• [x] Compute the pivot position of the vector data using the appropriate function.
• [x] Clone the subpaths in the vector data and mirror all of the positions around the computed pivot.

This will close #1453 when finished.

[Keavon]

Your code looks nice! However I need to get @TrueDoctor's comment about the addition of the pivot field, because this addition may likely conflict with his ongoing work to revamp the transform system. I'm not sure how to proceed with that so I'll wait for him to reply here.

[Keavon]

Blocked on #1558. We'll have to let this wait a bit. Hopefully @TrueDoctor can complete that issue in the next week or two.

Because it'll require a little bit of rework once that is merged (sorry for the trouble!), and because I want to make sure I can know which PRs require my attention, I'm going to mark this as a draft PR for now to help me stay organized. Apologies for the delay and extra effort this blockage, which isn't your fault, will incur.

[Keavon]

I think I'll close this for now. Thanks for your efforts, and I'm sure you learned a lot from this that'll be helpful knowledge as you work on the animation project this summer. But with the awkwardness of how transforms are currently (not) dealt with, it's rather hard to approach this at the moment. And there are lots of merge conflicts so closing for the moment is the easiest way to proceed. Thanks though!