Migrate seshat-node to Typescript

[Half-Shot]

This does a few things to migrate seshat-node to Typescript, and provide a type definition file.

This:

• Replaces index.js with index.ts, closely following the same types.
• Exports index.js / index.d.ts with a sourcemap included.
• Replaces eslint with oxlint/oxfmt as we would need to make eslint typescripty, and newer web packages are opting for these two packages instead.