package.json: recognize gopls.mod as a go.mod file

gopls.mod is a special go.mod file for a workspace used by gopls and friends.
Recognize this as a type of go.mod. For example, the language client library
will watch gopls.mod and trigger requests such as textDocument/*.

Note that vscode-go already issues 'workspace/didChangeWatchedFiles' for
gopls because gopls registers didChangeWatchedFiles capabilities on *.mod files.

Fixes #712

Change-Id: I47f8827dbaed8020ea34e356ccbb9422cf50932c
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/263986
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: Rebecca Stambler <rstambler@golang.org>

Author: hyangah

package.json

@@ -111,7 +111,8 @@
       {
         "id": "go.mod",
         "extensions": [
-          "go.mod"
+          "go.mod",
+          "gopls.mod"
         ],
         "aliases": [
           "Go Module File"