add type annotations

[trusktr]

Summary

Add some type annotations (#4789)

The behavior of the app shouldn't change (unless I goofed something), so there's nothing to test specifically, just that the app works.

Testing notes:

• if you use TypeScript and you open the source in your IDE, and your IDE is configured for TypeScript, this helps you see things such as informational tooltips when hovering on identifiers, autocompletion of possible values, parameter hints, etc.
• As far as OpenMCT itself, there is no change to behavior, nothing to test as far as functionality. If you haven't been using TypeScript intellisense, you'll notice no difference in your editing experience (apart from seeing additional comments in the source code, which you should strive to keep up-to-date if you modify any APIs).

[shefalijoshi]

@trusktr Please add some test instructions.

[unlikelyzero]

Testing notes:

if you use TypeScript and you open the source in your IDE, and your IDE is configured for TypeScript, this helps you see things such as informational tooltips when hovering on identifiers, autocompletion of possible values, parameter hints, etc.
As far as OpenMCT itself, there is no change to behavior, nothing to test as far as functionality. If you haven't been using TypeScript intellisense, you'll notice no difference in your editing experience (apart from seeing additional comments in the source code, which you should strive to keep up-to-date if you modify any APIs).

[unlikelyzero]

Will cover in demo day