Improve type signature for return value to be based on first arg

[sehrope]

Improves the type signature of the stringify function to take into account the main argument that we are serializing. If it is a primitive scalar, array, or object, and there is no second argument then the return value will always be a string.

As this is the common case for user applications, this improves the user experience as we do not have to check the result to ensure the value is defined. For most use cases where the callsite has a primitive, array, or object and is using the default serilaization options, the return value will be known to be a string

This likely addresses most real world issues with #19 as only those with custom options would get string | undefined as the return type.

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If you want to try this out, enable strict null checks in tsconfig.json and then try this:

import stringify from '../index';

const numberReturnsString: string = stringify(123);
const objectReturnsString: string = stringify({});
const object2ReturnsString: string = stringify({ foo: 123 });
const object3ReturnsString: string = stringify({ 123: "foo" });
const booleanReturnsString: string = stringify(true);
const nullReturnsString: string = stringify(null);
const arrayReturnsString: string = stringify([]);
const undefinedReturnsUndefined: undefined = stringify(undefined);
const unknownReturnsUnknown: string | undefined = stringify(undefined as any);