[super_text_layout] Add configurable strut style to SuperText

[larryisthere]

Summary

• Adds an explicit SuperText.strutStyle parameter.
• Adds SuperTextStrutStyle so an editor or text surface can provide a subtree-wide strut style without threading the value through every SuperText call site.
• Passes the selected strut style through LayoutAwareRichText to the underlying RenderParagraph.

Context

In a SuperEditor-based writing app, I needed stable line metrics while composing text with platform font fallback, especially in CJK/Latin mixed text. Exposing strut configuration at the SuperText layer lets clients opt in to a consistent line-height baseline without forking super_text_layout.

Tests

• flutter test in super_text_layout
• flutter analyze in super_text_layout