Insonsistent handling of the "browser" field

I have this in my package.json:

```
"browser": {
"./persistence.js": "./persistence-browser.js"
},
```

when I use `tsup --platform browser` it uses the browser version of the js, as it should. but with `--dts` it gets the types for the non-browser version.

Start by reproducing the issue with the shown package.json browser mapping, comparing tsup --platform browser with the generated declarations from --dts. Trace how the browser field is resolved for bundling versus declaration generation; done means the declaration types use the browser version consistently.

Written by the indexing model from the issue text.