refactor: migrate fetchXML to TypeScript and add tests

Resolves #309 (closed)

Resolves part of #381

This became a larger change than anticipated.

The original goal was to migrate fetchXML from JavaScript to TypeScript. However, it seemed like a good opportunity to drop jQuery here and to write unit tests to ensure the migration works as intended (and to increase coverage for the MBO.)

Then I required some API mocks for the xml endpoint from planeteclipse. The way I set mocks up was that the MSW mock servers run for the test suite under /api. This means I needed to move the fetchXML util to /api. Our utils file already had the UI utils and API utils separated by comments. Therefore, it would make sense to put API utils under /api and UI utils under /solstice.