chore: bump dependencies and migrate to pnpm

- Bumps the dependencies other than `openapi3-ts` to the latest versions
  as of this commit.
- Renames `rollup.config.js` → `rollup.config.mjs` because new Rollup
  does not allow ESM syntax in .js files outside the module.
- Migrates to pnpm.
- Bump version to 0.2.2.

Author: kikuomax