config: exclude test files from TS build output

[jamaljsr]

This is a follow-up to #123. I discovered an issue with the webpack build output. The index.d.ts file was being placed at dist/lib/index.d.ts instead of dist/index.d.ts, which is what the package.json defines as the entrypoint for types. This was because the TS test files, that were added in #123, were being included in the output. This is unnecessary and it causes errors when apps like Terminal import this package.

The fix is pretty simple. We just need to exclude *.test.ts files from the build output.

Before (wrong):

After (correct):

Steps to test

Just run yarn build and confirm that the dist/index.d.ts file exists and there are no *.test.d.ts files in the dist/dir.

[jbrill]

Simple change -- LGTM 👍