extension: start v0.42.0 dev cycle

For #3167

Change-Id: I279dcfd0d71ec75a0bf1cc1af5e19094ed13286e
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/564596
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
Commit-Queue: Hyang-Ah Hana Kim <hyangah@gmail.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: hyangah

extension/package-lock.json

@@ -1,7 +1,7 @@
 {
   "name": "go",
   "displayName": "Go",
-  "version": "0.41.0-dev",
+  "version": "0.42.0-dev",
   "preview": true,
   "publisher": "golang",
   "description": "Rich Go language support for Visual Studio Code",