package.json: update launch args schema

This is a follow-up change from golang#2670 to update the schema. This avoids intellisense warning squiggly lines when updating `launch.json` while using a `string` type for `args`.

Validated by importing the local `.vsix` extension, and verifying no warnings show up for `string` and `array` args.

Fixes golang#2621

Change-Id: I13248d65a2a13d4cf2b40921e7af4613ec3ce253
GitHub-Last-Rev: f6fefc3
GitHub-Pull-Request: golang#2777
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/495816
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
TryBot-Bypass: Hyang-Ah Hana Kim <hyangah@gmail.com>
Commit-Queue: Hyang-Ah Hana Kim <hyangah@gmail.com>

package.json

@@ -678,7 +678,7 @@
                 "default": false
               },
               "args": {
-                "type": "array",
+                "type": ["array", "string"],
                 "description": "Command line arguments passed to the debugged program.",
                 "items": {
                   "type": "string"