[release] package.json: v0.28.1

Update version string for release

Change-Id: I06bdbd26e6623bfb22a062264b7f3d1d90e6ca47
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/352053


Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>