Bugfix. Properly set exports value in package.json

[jcbrand]

Otherwise the following error appears when importing from Strophe:

Module not found: Error: Package path . is not exported from package

From what I can tell looking at the documentation (https://webpack.js.org/guides/package-exports), the previous values are invalid since they don't start with a ., so I removed them.

Leaving them in also caused this error:

Module not found: Error: Exports field key should be relative path and start with "." (key: "node")