Prompt transform parameter input while adding tags to struct

In the current version, Go: Add Tags prompts to get user input for only tags and options if go.addTags.promptForTags is true. With the following changes, it will also prompt for transform value too.

Closes [the request](https://github.com/microsoft/vscode-go/issues/2546)

![80874408-270e9580-8cc3-11ea-9d1d-b56f9b2da28d](https://user-images.githubusercontent.com/58530683/82475027-8696de80-9ad4-11ea-97cf-e2b48468db25.gif)

Change-Id: Ib27c6cef17d64b782095994e12e7e75133a28a89
GitHub-Last-Rev: 79a164d4118c9ea34956eda61e2f10d8f11b8476
GitHub-Pull-Request: golang/vscode-go#78
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/234659
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/src/goModifytags.ts b/src/goModifytags.ts
index 707ae5b..ac25e88 100644
--- a/src/goModifytags.ts
+++ b/src/goModifytags.ts
@@ -128,7 +128,14 @@
 					prompt: 'Enter comma separated options'
 				})
 				.then((inputOptions) => {
-					return [inputTags, inputOptions, transformValue];
+					return vscode.window
+						.showInputBox({
+							value: transformValue,
+							prompt: 'Enter transform value'
+						})
+						.then((transformOption) => {
+							return [inputTags, inputOptions, transformOption];
+						});
 				});
 		});
 }