[Feature Request]: Add right-click upload/verify from Explorer

[JulianOrteil]

This is an extension of #382, #481 and #915 to allow uploading/verifying of different sub-projects within the same root folder.

I offer this suggestion as a work around to this concern #382 (comment) and incorporates the ideas suggested by #915.

If #915 is possible, I believe setting the sketch setting in arduino.json to null should be the default behavior. This would emulate the behavior of other extensions, like Python, which defaults to the current opened file when a debugging session is started via the button seen below. I expect new users would prefer this behavior over the current to remain consistent with larger extensions.

To incorporate #382 and expand on #481, I propose the following:
When a user right-clicks a .ino file in the Explorer, two new options should be available in the context menu displayed. They should be along the lines of "Upload Sketch" and "Verify Sketch". Instead of possibly requiring build tasks as suggested by #382 (comment), the extension should instead pull all configurations from the arduino.json file except for the sketch entry. The path to the right-clicked sketch should be used instead of the sketch entry in arduino.json. Then the task requested should be executed.

From what I can already see in the extension, I believe much of the groundwork for this proposal is already available in different modules.

Please share any of your thoughts on this and offer concerns or suggestions to improve it.