maxsize/minsize refactor away from percents

[dginev]

Follow-up to w3c/mathml-core#103

The group has clarified that the percentage minsize/maxsize value is relative to the "normal stretched size" of a stretchy <mo>, which is contextual to the size of its parent.

For cases where latexml tries to size w.r.t to the normal font size of a glyph (e.g. \small), we should consider switching away from percentage values such as 90% and closer to relative unit values, such as 0.9em.

We would still benefit from tests and interop in browsers, since the 0.9em values may still have lingering rendering issues.

[dginev]

This was resolved by #2341 , closing.

(Although it is unclear if we want to revert at some point...)