Fix application of fontFamily to child nodes

[amake]

Currently, applying a different fontFamily to a nested node doesn't work:

Html(
      data: 'Hello, <em>world</em>',
      customTextStyle: (node, style) {
        if (node is dom.Element) {
          switch (node.localName) {
            case 'em':
              return style.copyWith(fontFamily: 'DM_Serif_Display'); // or whatever
          }
        }
        return style;
      },
    )

One would expect world to be shown in the specified font, but it isn't. (It is correctly displayed in italics, though.)

The cause is, apparently, this: flutter/flutter#35992

The workaround is easy, but yucky. I personally can't wait for the core issue to be fixed and the fix to make its way to the stable channel, so I will just use my fork, but I understand if you'd prefer to wait for this to be fixed upstream rather than merge this.

[Sub6Resources]

Thanks for the PR. I'm going to wait a week or two to see if there's any progress towards resolving that Flutter issue you mentioned. If there isn't I'll consider merging this pr.

[amake]

What are your thoughts on merging this? My concerns are:

1. The real fix will take a long time to get out
2. I have a feature PR I'd like to open that I currently have implemented on top of this fix, so I am blocked on your decision at the moment. If you'd like to pass on this fix, I will redo it on master.

I'm happy either way, so please let me know.

[amake]

The core fix appears to have made it to the beta channel now (1.8.3): flutter/flutter@c15cfdd

That's actually good enough for me. Shall I close this PR?