feat: allow overriding text style for rich text

[CazSaa]

This PR adds functionality to override TextStyle for RichTextLeafMarkable.

[PhiFry]

Thank you for your submission! Do you want to provide a specific change that you needed for your use case? For example to change the font or color etc. Just so I can get an idea of what users of this library might want to customize in the future. Thank you.

[CazSaa]

We used it like this:

textStyleOverrides: (RichTextLeafMarkable leaf) =>
    TextStyle(
  color: CustomTheme.primaryColor,
  fontWeight: leaf.isBold || leaf.link != null
      ? FontWeight.bold
      : FontWeight.normal,
)

to set our own text colour, and to make links bold.

[PhiFry]

Its a simple solution for the problem. Code looks good 👍