package.json: prepare for the v0.40.0 dev cycle

Fixes #2798

Change-Id: I9af80552f5c6767e202e36472c31d88c9aad5505
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/505215
TryBot-Result: kokoro <noreply+kokoro@google.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>

Author: hyangah

package-lock.json

@@ -1,7 +1,7 @@
 {
   "name": "go",
   "displayName": "Go",
-  "version": "0.39.0-dev",
+  "version": "0.40.0-dev",
   "preview": true,
   "publisher": "golang",
   "description": "Rich Go language support for Visual Studio Code",