Excluding files from the build

Hello

How can we exclude files from compilation (for example test files ending with .spec.ts)? I tried https://github.com/egoist/tsup/issues/415 but it errored:

```
ESM Build failed
Error: Build failed with 1 error:
error: The entry point "./src/api.spec.ts" cannot be marked as external
```

Then I switched from using `npx tsup src` to `npx tsup src/index.ts` and I got an error that other files are imported but are not entrypoints.

```
src/index.ts(1,15): error TS6307: File '/src/endpoints.ts' is not listed within the file list of project ''. Projects must list all files or use an 'include' pattern.
The file is in the program because:
Imported via './endpoints' from file '/src/index.ts'
Imported via './endpoints' from file '/api.ts'
src/index.ts(3,15): error TS6307: File '/src/api.ts' is not listed within the file list of project ''. Projects must list all files or use an 'include' pattern.
```

Then I tried to "include" everything under "./src" in tsconfig.json, but it didn't help.

Then I tried to use the glob library to exactly specify my import files and exclude test files, but I don't think tsup supports asynchronous configs?

Is there a way to exclude files within the src directory (preferably without creating a node script that calls tsup's "build" directly)?

Reproduce the reported `npx tsup src` and `npx tsup src/index.ts` commands with the described `tsconfig.json` setup, then inspect how tsup handles entry files and imported files. Done means providing a supported way to exclude `*.spec.ts` files under `src` without requiring a custom build script.

Written by the indexing model from the issue text.