Add all named endpoints and types

[aoberoi]

This change adds named methods to the Client class for each of the endpoints in the Notion API. It follows the pattern established in #1.

Fixes #13

Future work

It's clear that the type definitions are not ideal. I intentionally avoided complex type constructs such as conditional types and heavy use of generics because the eventual goal is to generate these types from an OpenAPI format. If we used these more complex types now, it's very unlikely we could generate something which kept stability in the type names. TypeScript is just more expressive than OpenAPI and JSON Schema.

Another shortcoming is that there isn't a good separation between "input" types and "output" types. This matters in situations where a method accepts, for example, a Block in its parameters. At call time, a block won't have properties like id - those are assigned by the server. However, the Block type has id set as a non-optional property. This is a real issue and should be handled in a bugfix.