Ignore the .ionide working folder (#3108)

If you have the [Ionide](http://ionide.io/) tools installed you will get an `.ionide` directory created in each directory that you open with VS Code, regardless of whether or not you are using F#.

VisualStudio.gitignore

@@ -347,3 +347,6 @@ healthchecksdb
 
 # Backup folder for Package Reference Convert tool in Visual Studio 2017
 MigrationBackup/
+
+# Ionide (cross platform F# VS Code tools) working folder
+.ionide/