Function font_synthesis 

pub fn font_synthesis(
    child: impl IntoUiNode,
    enabled: impl IntoVar<FontSynthesis>,
) -> UiNode

P Configure if a synthetic font is generated for fonts that do not implement bold or oblique variants.

Not all fonts implement the requested font_weight and font_style, this config allows the renderer to try and generate the style and weight anyway, using transforms and the glyph outlines.

Sets the FONT_SYNTHESIS_VAR.