[WIP] Update Type Definitions and API Docs to Make Sure They Match

[mdemichele]

I'm trying to go through the Type Definitions and API docs to make sure that they're up to date and fully documented. So far, here are the ones I've found:

• Adds missing deserialize property to PersistConfig type interface. Updates the APIs docs accordingly. Mentioned in original repo here: #1443

[mdemichele]

Apologies if these are dumb questions, but since I'm not fully an expert in typescript, I need to be honest about gaps in my knowledge and ask a few questions:

• In the old repo, I saw that the /types folder was added to the .gitignore. But, the older maintainers still left the existing /types folder in the repo. If we're not going to continuing keeping track of changes in the /types folder, shouldn't it be removed from the repo completely? From what I can gather about typescript, it seems like the types.d.ts file inside of the /types folder was auto-generated from the Typescript compiler based off the settings in tsconfig.json and any .ts files the compiler found in the /src folder. If that's the case, then there's not really a reason to keep the contents of the /types folder around. Am I right in that understanding?
• There wasn't really much of a review process before the original PR went in converting everything to TypeScript. It'd probably be a good idea to revisit that change and make sure everything is set up properly. A couple of questions I have about that PR are: Is there anything in the tsconfig file that we need to change? Are there any places in the /src folder that haven't been fully converted to TypeScript yet? If not, this PR would be a good place to find those leftover things that still need conversion to typescript.

[mdemichele]

I changed this to a string type to match what's in the types.ts file, but I'm wondering, is number correct here? If you look at the migrations.md example, the keys are numbers. So, is the types.ts file wrong or is the api.md and migrations.md files wrong?