Switch webpack to submodule (microsoft#23372)

* Prepare for adding webpack submodule

* Actually switch webpack to submodule

I'm sure this will work with no problems!

* Add ignore=all to webpack gitmodule entry

The other submodules do this, although I'm not sure why.

* Add newline at end of .gitmodules

Author: sandersn

.gitmodules

@@ -18,3 +18,7 @@
 	path = tests/cases/user/TypeScript-WeChat-Starter/TypeScript-WeChat-Starter
 	url = https://github.com/Microsoft/TypeScript-WeChat-Starter.git
 	ignore = all
+[submodule "tests/cases/user/webpack/webpack"]
+	path = tests/cases/user/webpack/webpack 
+        url = https://github.com/webpack/webpack.git
+	ignore = all

tests/baselines/reference/user/webpack.log

@@ -0,0 +1,3 @@
+{
+    "types": []
+}