File loader doesn't work for .md files

I want to copy readme.md file to `/dist` so entire folder can be submitted to npm

Repo where this can be reproduced:
https://github.com/saleor/saleor-app-sdk/blob/main/tsup.config.ts

1. Add `README.md` to `entries`
2. Add `loader: { '.md': 'file' }` to config

## Expected
file will be copied to dist

## Actual

Error
```
DTS Build start
error TS6054: File '/Users/lukasz/projects/saleor-app-sdk/README.md' has an unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts', '.cts', '.d.cts', '.mts', '.d.mts'.
The file is in the program because:
Root file specified for compilation
```

Adding `*.md` to tsconfig `excludes` doesn't change anything

Reproduce the issue using the linked tsup.config.ts with README.md in entries and loader: { '.md': 'file' }, then inspect how the DTS build handles those entry files and the tsconfig exclusion. Done means the markdown file is copied to dist without the reported TS6054 error.

Written by the indexing model from the issue text.