package.json: add missing transform variants for go.addTags

This simply allows in the settings.json or UI to use the missing 'lispcase', 'pascalcase' and 'keep'
transform options for the go.addTags command.

fixes #906

Change-Id: Ic1ef1fd6e9df755d048424275e75033914c91060
GitHub-Last-Rev: e181a58
GitHub-Pull-Request: #936
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/271357
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>

Author: skaldesh

package.json

@@ -1689,7 +1689,10 @@
               "type": "string",
               "enum": [
                 "snakecase",
-                "camelcase"
+                "camelcase",
+                "lispcase",
+                "pascalcase",
+                "keep"
               ],
               "default": "snakecase",
               "description": "Transformation rule used by Go: Add Tags command to add tags"