Feature/add font family fallback

[kaiquegazola]

Add fontFamilyFallback param to Style and pass it to TextStyle.

Usage:

Html(
      data: '<span>‟Pessoas que se recuperaram, »’”›⸃⸅⸊⸍⸝⸡ [...]”</span>',
      style: <String, Style>{
        'span': Style(
          fontFamily: 'Lato',
          fontFamilyFallback: ['Arial', 'Roboto'],
        ),
      },
);

and can render characters that do not exist in the main fontFamily (and exist in any of fallback fontFamily, in order, of course):

This pr can close #1078

[erickok]

Looks good to me. thanks for the contribution. What do you think @Sub6Resources ?

[Sub6Resources]

LGTM. Fairly straightforward, thank you! My only thought is that in CSS, font-family is one attribute that handles both of these cases. This looks good, and I'll add and take on an issue to create tests to ensure the css parser correctly splits font-family into our fontFamily and fontFamilyFallback attributes.