AX subrole of <mo> element

[fred-wang]

cc @asurkov

Currently, it's defined to always be "AXMathOperator".

However, if the <mo> element has the "fence" or "separator" property, then WebKit specializes this to "AXMathFenceOperator" and "AXMathSeparatorOperator" respectively:

https://webkit-search.igalia.com/webkit/source/Source/WebCore/accessibility/ios/WebAccessibilityObjectWrapperIOS.mm#3068

MathML Core does not define any special behavior for these properties, however it defines the corresponding attributes as well as table of operators having these properties by default:

https://w3c.github.io/mathml-core/#dfn-fence