content: **bold text** should be formatted with `weightVariableTextStyle`

[chrisbobbe]

Like other text that we want to present in Source Sans 3 (a variable-weight font), we should style **bold text** in content with weightVariableTextStyle. Currently, it is styled with just fontWeight:

  InlineSpan _buildStrong(StrongNode node) => _buildNodes(node.nodes,
    style: const TextStyle(fontWeight: FontWeight.w600));

[gnprice]

Writing down a discussion we had in the office: it looks like the symptoms of this bug are currently subtle, but only because of the upstream Flutter issue discussed at #500.

The TextStyle that ultimately applies to the text inside one of these spans inherits a fontVariations from an ancestor, which in a paragraph is going to be the one weightVariableTextStyle gives for normal weight. In the absence of that upstream issue, the wght variation from there would control (probably, depending on how the issue gets resolved), and the text would have normal weight (on default system settings) despite being from a StrongNode.

But with that issue, the fontWeight combines with the wght variation and we get a bold effective weight. Effectively this bug roughly cancels out with #500 (… at least on Android, where #500 applies). I say "roughly" because even in the scenario where they both trigger, the effect of #500 seems to make the glyphs heavier but leave their spacing unchanged, so it's not quite the same effect as getting the actual bolder version of the font.