Implement the context menu for Variables view

[zero-plusplus]

Implement this feature added in 1.49.

This will solve the following problems suggested in #20

2. I want to copy and paste the value so it pastes correctly (preferably without the encapsulating quotes) e.g.
   When I click on value in VARIABLES Pane, I don't want to see the quotes:

If implement this feature, the required version of VSCode will be changed from 1.45 to 1.49, so need to increment the major version of this extension (i.e. 2.x.x)
Enough time has passed since the release of vscode 1.49 that no major version increments are needed.