Add typesaurusSettings function with undefinedToNull setting

[SrBrahma]

Based on #100,

Now it's exported typesaurusSettings(settings: Partial<TypesaurusSettings>): void, with type TypesaurusSettings = { undefinedToNull: boolean; }

To keep its current behavior, undefinedToNull defaults to true.

To use it, simply call typesaurusSettings({undefinedToNull: false}) before write operations are made in your code.

[kossnocorp]

Thank you for the PR, sorry for not merging it but the problem got addressed in v10: https://typesaurus.com/decisions/adr-1-undefined/