Migrate to tsdown for bundling

[bkeepers]

The .d.ts files published by vite are not transforming the json imports, leading to this error:

../../node_modules/@neaps/tide-database/dist/index.d.ts:47:41 - error TS2307: Cannot find module './constituents.json' or its corresponding type declarations.

47 export { default as constituents } from './constituents.json';
                                           ~~~~~~~~~~~~~~~~~~~~~

This replaces vite with tsdown for bundling, which does the right thing with the json imports. This also adds a CJS build, which neaps currently supports.