When specifying a folder to compile, empty js files are output for `.d.ts` declaration files

[bbugh]

Given a file setup that functions:

src/main.ts
src/types.d.ts

When running sucrase like this:

npx sucrase ./src -d ./dist --transforms typescript

Then the dist folder contains:

src/main.js
src/types.d.js

And the src/types.d.js contains only whitespace.

Expectation: d.ts files should read as part of the TypeScript processing, but should not be output.