extension/package.json: edit default launch package dir

Edit .vscode/launch.json `Go: Launch package` template from `"program": "^\"\\${fileDirname}${1:}\""` to `"program": "^\"\\${workspaceFolder}${1:}\""`

Fixes #3387

Change-Id: I35af5ebe551830c2b99c62c652e6ad7309a0527b
GitHub-Last-Rev: d8df56c
GitHub-Pull-Request: #3388
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/585615
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
Commit-Queue: Hyang-Ah Hana Kim <hyangah@gmail.com>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>

extension/package.json

@@ -554,7 +554,7 @@
               "type": "go",
               "request": "launch",
               "mode": "auto",
-              "program": "^\"\\${fileDirname}${1:}\""
+              "program": "^\"\\${workspaceFolder}${1:}\""
             }
           },
           {