Convert CaseStyleConverterBag, MathResolver, and NameSplitter to standalone functions and variables

[JoshuaKGoldberg]

It's annoying having to constantly do things like this:

import { convertToCase } from "???/CaseStyleConveterBag";

const caseStyleConverterBag = new CaseStyleConverterBag();

caseStyleConveterBag.convert(...);

These classes are exclusively string manipulations and aren't going to need local state. Now that standalone functions are supported, they should be changed to that easier API:

import { convertToCase } from "???/CaseStyleConveterBag";

convertToCase(...);