Added support for Constant values that can be reused in the stylesheet

[lucaslt89]

• Migrated to Swift 5. Made StyleableComponents a public header
• Added support for constant values in the JSON file defined in a new object at the same level of the styles object

    "constants": {
        "PrimaryColor" : "#FF5A5F",
        "Header5Font" : {
            "name": "CircularStd-Bold",
            "size": 18.0
        }
    }

That can be later reused in the styles dictionary as following:

        {
            "styleName": "Primary",
            "styleProperties": {
                "backgroundColor": "$PrimaryColor"
            }
        },
        {
            "styleName": "Header5",
            "styleProperties": {
                "font": "$Header5Font",
                "textColor": "#3f444c"
            }
        }

[daniel-hall]

@lucaslt89 This is a great addition — thanks for the PR! Only two requests:

1. See other comment inline — I'm concerned about breaking existing stylesheet json files with the change and a solution to avoid that (for example, making the constants dictionary optional) would be needed.

2. How about updating the StylishExample project in this repo so the example of the stylesheet.json uses a constant as supported by your addition, perhaps for a color since there are some duplicates in there IIRC?

Thanks again for proposing this useful addition!

[daniel-hall]

Thanks for the updates! Glad to have this new feature!