Hi!
As per documentation it should be possible to pass inline as sourcemap option. In reality, passing it is treated the same as passing any other value thanks to this line: https://github.com/egoist/tsup/blob/dev/src/esbuild/index.ts#L134

Moreover, the relative test is wrong too and doesn't catch this error. We can see that it is checking for the presence of
at https://github.com/egoist/tsup/blob/dev/test/index.test.ts#L188 instead of the more safe //# sourceMappingURL=data:

I've already had a fix, and I can open a PR, but there is just one last thing that bothers me. esbuild documentation says that external shouldn't add //# sourceMappingURL= at the end of the file, but running this test and checking the output, shows a different story. Any clues?

And this is why the inline test is passing even if the option is not working at the moment.

...
PERFORMANCE OF THIS SOFTWARE.
***************************************************************************** */
//# sourceMappingURL=input.js.map