Added signal declaration to the snippets

[GlenNicholls]

I need to review this and compare it to other snippets. I’m somewhat okay, but I need to go through everything I wrote again to make sure it makes sense to add one for a signal declaration and ensure this is consistent

[GlenNicholls]

This IMO should be more descriptive. For example, look at the following:

"Alias": {
        "prefix": "alias",
        "body": [
            "alias ${1:<name>} : ${2:<subtype>} is ${3:<object_name>};",
            "$0"
        ],
        "description": "alias declaration"
    },

It'd be nice if you also replicated signal but for variable as well.

[GlenNicholls]

I'm not allowed to approve this, so you might want to ping the maintainer.

[kraigher]

@GlenNicholls I gave you write permissions to the repository if you want to make these kind of changes.