Standardize TypeScript usage

[victorlin]

Now that TypeScript is being used more in this codebase, we should standardize usage.

Consider enforcing with ESLint:

• [ ] Typing function return calls via @typescript-eslint/explicit-function-return-type
• [x] Avoid non-null assertions: ef18825f9ee581b70959726822c6a90788cf62ae
• [x] Avoid type assertions: 4592e2a983ef039fba742c4c5b8c807d9446df71

Other things that can be added as guidelines in DEV_DOCS:

• Usage of unknown vs. any
• When to put types in a separate types.ts file
• type vs. interface for object types
• Defining functions for type narrowing