Test mono-debug installation hint

[isidorn]

• OS X @seanmcbreen
• Linux @joaomoreno

We plan to make mono-debug an optional extension. To make a smooth transition we let the user know once and offer a direct install. Have a launch.json with type mono. Verify:

• you are informed and offered to install of mono-debug
• the hint only shows once
• you can nicely install mono-debug from the hint and debugging works after the install

[seanmcbreen]

I did this with the unity integration and worked well.

[joaomoreno]

@seanmcbreen u forgot the checkbox?