Add custom_shapes.js, which will eventually replace vertex.js

[GregStanton]

Addresses #6560

Changes:

This is a small PR that just sets up a new file, with the following changes:

1. Adds shapes -> custom_shapes.js, which is intended to eventually replace vertex.js
2. Registers the add-on in shapes -> index.js.
3. Updates src -> app.js so that the new features are included there.

This was originally intended as a draft PR so that I could continue adding features, but I accidentally messed up the two commit messages. So I'm thinking maybe this PR can be squashed and merged, with the PR title as the commit message? Then I can submit a new PR with any additions?

Note:
Once we're ready to remove the old implementation, I propose that we keep the file name custom_shapes.js (instead of vertex.js) and use the same name for the submodule, so that "Custom Shapes" appears as a heading in the reference instead of "Vertex." The new name is general enough to encompass all the functions it contains, while also being more descriptive for beginners. The phrase "custom shapes" already appears in descriptions of the relevant functions as well, so it's consistent with existing reference descriptions.

PR Checklist

• npm run lint passes