Fix two omissions from #19973 that prevented `twenty-sdk/billing` from
being a fully exported subpath:
- `package.json`: add `billing` to `typesVersions` (every other subpath
was listed; billing was the only one missing, breaking type resolution
for consumers using classic TS moduleResolution).
- `project.json`: add the billing vite build and `dist/billing` output
to the `build:sdk` target