Implement the `git stash` command.

`git stash save` = Stash: Save Changes
`git stash pop` = Stash: Apply (Pop)
`git stash apply` = Stash: Apply (Keep)
`git stash drop` = Stash: Delete
Add 'Drop' to Git Stash Delete in command menu
Merge branch 'git-stash'
Update Changelog.md
Prepare 3.8.0 release