Add declaration file to module.assets

[dmitriy-tarasov]

Hi,

I see that emitting d.ts files through this.emitFile was removed in #175 PR and in result I can't access them via module.assets object. That was very convenient to know which exactly ts file was producing appropriate .d.ts.
Is it possible to turn it back?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Closing as stale. Please reopen if you'd like to work on this further.