Also update the extension version in package.json.

Change-Id: I1d9167879f6b9b7b241b56ddc6a4b68e2f7170a9
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/246998
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>