Change how the `Database` object is generated

[aleclarson]

Currently, Rollup cannot treeshake db.xyz access, because ES6 classes are not treeshaked.

Instead of there being a Database class with the generated client having a singleton instance, do the following:

• Move all commands into plugin dependencies of the tusken package, so *default commands* can be tree-shaked.
• Emit database extensions with export const syntax instead of using a subclass, so plugin methods can be treeshaked if not used in a bundle.
• Refactor database state into a POJO stored in a global variable. Then make this POJO the default database context of each database method.
• Now move the database state & methods from ./index.ts to ./database.ts
• Inside the ./index.ts module, do this instead:

  export * as db from './database'
  export * as pg from './functions'
  export * as t from './types'