style: Implement displaystyle attribute on all MathML elements.

Gecko implements the displaystyle attribute as defined by MathML 3 i.e.
only on the math, mstyle and mtable elements. This commit follows MathML
Core and instead define it as a global attribute mapped to math-style.
See https://mathml-refresh.github.io/mathml-core/#the-displaystyle-and-scriptlevel-attributes

Differential Revision: https://phabricator.services.mozilla.com/D90786

components/style/properties/longhands/font.mako.rs

@@ -247,6 +247,7 @@ ${helpers.single_keyword(
     has_effect_on_gecko_scrollbars=False,
     animation_value_type="none",
     enabled_in="ua",
+    needs_conversion=True,
 )}
 
 ${helpers.single_keyword(