fix: Handle 'pixel' size mode for shape labels

[camdecoster]

Description

Update shape label positioning logic to handle pixel size mode.

Closes #7749.

Changes

• Update position calculations
• Export helper to aid in calculations
• Add tests
• Add mock with baseline image

Screenshots

Before	After

Testing

• Be on master
• Run devtools
• Add new mock temporarily on branch
• Load the mock
• Note that the labels are missing
• Switch to this branch
• Load the mock again
• Notes that the labels are positioned correctly

[camdecoster]

The initial mock validation failed because one of the shape types was listed as "rectangle", which isn't valid. "rect" is what should have been listed. I didn't notice this because the default shape used when an invalid type is passed in is... "rect". Thanks CI, for keeping us honest.