feature/subtask getters

[joshua-dean]

Add getters for current subtask and subtask key

Description

• Added .get_current_subtask_key() and .get_current_subtask() on ULabel main object
• Replaced over 200 instances of previous access pattern

Closes #183.

There's a lot of instances where const subtask is declared when it really should be const subtask_key. I can open an issue to remedy this, or try to tackle all of them in this PR. Let me know.

PR Checklist

• Merged latest main
• Version number in package.json has been bumped since last release
• Version numbers match between package package.json and src/version.js
• Ran npm install and npm run build AFTER bumping the version number
• Updated documentation if necessary (currently just in api_spec.md)
• Added changes to changelog.md

Breaking API Changes

[TrevorBurgoyne]

LGTM, as far as i can tell everything still works as expected