[WIP] Add command for creating a new Cargo project

[GabrielMajeri]

This PR adds a new command which asks the user for some details then creates a new crate using cargo new.

Fixes #254.

Things left to do:

• Move the createNewProject command implementation somewhere else. I haven't yet figured out where to place it.
• Add tests. I'm not familiar with VS Code extension development so I don't exactly know how this would be implemented. I'm guessing I'd add a new function in extension.test.ts.
• Refactor the way user input is checked. Right now, after every input box I have a statement like if (!crateType) { return; } which just aborts the project creation process if the user clicks outside the box without providing an input. I don't know if this could be solved in a less repetitive way.