Add VS Code snippets for Bevy

[benfrankel]

Fixes #202.

[janhohenheim]

Many of these could be postfix instead, but I have no strong feelings on that.
Does adding these preserve the original custom Rust snippets? I know that they can get overridden pretty easily if you're not careful. For reference, these are the builtin ones:

       "Arc::new": {
            "postfix": "arc",
            "body": "Arc::new(${receiver})",
            "requires": "std::sync::Arc",
            "description": "Put the expression into an `Arc`",
            "scope": "expr"
        },
        "Rc::new": {
            "postfix": "rc",
            "body": "Rc::new(${receiver})",
            "requires": "std::rc::Rc",
            "description": "Put the expression into an `Rc`",
            "scope": "expr"
        },
        "Box::pin": {
            "postfix": "pinbox",
            "body": "Box::pin(${receiver})",
            "requires": "std::boxed::Box",
            "description": "Put the expression into a pinned `Box`",
            "scope": "expr"
        },
        "Ok": {
            "postfix": "ok",
            "body": "Ok(${receiver})",
            "description": "Wrap the expression in a `Result::Ok`",
            "scope": "expr"
        },
        "Err": {
            "postfix": "err",
            "body": "Err(${receiver})",
            "description": "Wrap the expression in a `Result::Err`",
            "scope": "expr"
        },
        "Some": {
            "postfix": "some",
            "body": "Some(${receiver})",
            "description": "Wrap the expression in an `Option::Some`",
            "scope": "expr"
        }