Use TypeScript readonly modifier for external/array types

[bmish]

Use TypeScript readonly keyword in many places. This prevents us from modifying certain properties/data, kind of like the ESLint no-param-reassign rule. It's best practice to avoid modifying input data like user-provided options.

• Use readonly keyword on all internal array types
• Use readonly keyword on all our public GenerateOptions option types
• Perform a few minor refactorings to avoid mutating affected data

If using readonly ends up causing any issues, we can relax some of this later.

https://www.typescriptlang.org/docs/handbook/2/classes.html#readonly
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html#a-new-syntax-for-readonlyarray