Define MathML features with BCD

[foolip]

In #976 I took a shortcut because sorting through the long history of MathML seemed like a very big task, and I wanted to unblock #890. This comment explains it:

web-features/feature-group-definitions/mathml.yml

Lines 5 to 16 in 344dd39

	# MathML has evolved for more than 20 years and computing its status from BCD is
	# challenging. To represent basic support for MathML, the following statuses
	# match the earliest support claimed by caniuse.com or BCD:
	# https://caniuse.com/mathml
	# https://developer.mozilla.org/en-US/docs/Web/MathML/Element/math
	#
	# Note that there are minor disagreements between sources about initial support
	# in different browsers. Where either source claims partial support, the
	# versions were increased. Following this method, Firefox was increased due to
	# BCD, and Safari due to caniuse.com.
	#
	# TODO: Partial support in Firefox <4 and Safari <10, with notes.

Because of this, a Baseline banner won't be shown on any MDN page about MathML.

Defining MathML in terms of BCD will require pinning down the initially supported subset of MathML, and the difficulty for me is that I don't trust that BCD is actually correct about the exact version numbers that far back.