Add a menu item for "New VDM Project"

[nickbattle]

Currently, new VDM projects have to be made by creating a folder by hand and opening it (or including it in a workspace) and then manually creating source files and .vscode files. A VDM-specific wizard, similar to other language extensions, would be much better.

The Java extension does this by adding a main menu item, as well as a button on the "Nothing open" panel. The wizard offers templates as well as the location of the new project folder and its folder name. Following the same style for VDM would be familiar to users.

[c4rlo14]

We considered several approaches here, but eventually we decided to follow the VS Code Java extension's New Java Project style, as it may already be familiar to VS Code users and fits naturally into the platform.

The new vdm-vscode.createProject command is accessible from the explorer welcome panel (when nothing is open), the right-click context menu, and the command palette. It walks the user through dialect selection, template selection, parent folder and project name. It finally offers to open the new project standalone or add it to an already-open workspace (if any). Three initial templates are provided, one per each dialect. Each template has a skeleton source file and a starter launch.json.