Support nodenext Typescript moduleResolution when using codegen

[ryscheng]

When we use codegen today, the generated files will omit the file extensions when importing files.

However, new ESM modules in TS require relative imports to specify the full file extension
See
https://www.typescriptlang.org/docs/handbook/esm-node.html

relative import paths need full extensions (e.g we have to write import "./foo.js" instead of import "./foo")