Build initial test suite

[aedwardg]

Purpose

To incorporate an automated test suite for unit testing.

Is your feature request related to a problem? Please describe.

We're currently manually testing everything. This is a long, tedious, and error-prone process.

Proposed Solution

Build out an initial test suite with TypeScript and Jest.
I suggest using TypeScript here to make it easier to mock out Discord.js, and Jest because it has built-in mocking, coverage, and many other features without the need to add third-party libraries.

Acceptance Criteria

Please list the requirements the implementation of this feature should meet.

• Tests are organized, readable and test the correct aspects of our bot
• Tests can be run automatically by GitHub Actions

[aedwardg]

Tabling this for now, since most of the tests I have written probably won't work w/v13+slash commands