add a bin/webpack (#225)

### Summary
see:
Betterment/test-track-app#31 (comment)

In CI we have a frontend build verification step which runs either
`bin/vite` or `bin/webpack`, but `test_track` uses none of those. As a
workaround, this commit adds a `bin/webpack` script that just runs `yarn
build:css` internally.

### Other Information

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/domain @Betterment/test_track_core

bin/webpack

@@ -0,0 +1,3 @@
+#!/usr/bin/env sh
+
+yarn build:css