Add startvs for each solution (dotnet/extensions#2630)

\n\nCommit migrated from https://github.com/dotnet/extensions/commit/e41d90f66109f3387063f412a0ec61c85f8ecbdb

src/JSInterop/startvs.cmd

@@ -0,0 +1,3 @@
+@ECHO OFF
+
+%~dp0..\..\startvs.cmd %~dp0JSInterop.slnf