Fix theme color not taken into account when a style is present

[nicolasbraun]

Motivation

Currently if a theme is set the link colors are not properly taken into account because we regenerate a MarkdownComponent from config, which is child of the TextSpan and override the Textspan's style

Switching to a simple TextSpan seems enough but otherwise we need to do something like this

    var theme = GptMarkdownTheme.of(context);
    final style = config.style?.copyWith(
      color: theme.linkColor,
      decorationColor: theme.linkColor,
    );
    final newConfig = config.copyWith(style: style);
    var linkTextSpan = TextSpan(

      children: MarkdownComponent.generate(context, linkText, newConfig, false),
      style: style,
    );

Note i also set the decorationColor to the style, to follow the logic in LinkButton.

Notes on link customization

I feel link layout customization is a bit edgy:

• if there is no config.style then the theme colors are completely ignored. Maybe we should detfault to TexStyle() (same logic as LinkButton) (or simply not pass the child to linkButton and let him handle it ?)
• If one want's another TextStyle for links like bold it's not possible without a custom builder, maybe an optional linksTextStyle in config is a good idea
• If there is a config.style, then link do not have underline (since we pass child to LinkButton)
• Maybe the linkDecoration and linkDecorationColors should also be in the Theme