Change definition of space-like maction and semantics

[fred-wang]

Defintion https://mathml-refresh.github.io/mathml-core/#dfn-space-like has the following:

• Or an unknown MathML element, <mrow>, <mstyle>, <mphantom> or <mpadded> all of whose in-flow children are space-like;
• An <maction> or <semantics> element whose first in-flow child exists and is space-like.

Test: mathml/presentation-markup/spaces/space-like-002.html

When trying to make maction/semantics behave as mrow in https://bugzilla.mozilla.org/show_bug.cgi?id=1588733 ; this test now fails.

Proposal: merge these two items:

• Or an unknown MathML element, <mrow>, <maction>, <semantics>, <mstyle>, <mphantom> or <mpadded> all of whose in-flow children are space-like;

Since normally non-visible maction/semantics children are displayed "none", they are effectively out-of-flow and the MathML3 logic is preserved.

[davidcarlisle]

This seems reasonable to me

[fred-wang]

Consensus from 2019/11/11: Handle maction/semantics the same as other mrow-like ("whose in-flow children are space-like" instead of "whose first in-flow child exists and is space-like")

[NSoiffer]

@fred-wang: can this be closed due to #182 and #183?

[fred-wang]

Tests updated for the new definition web-platform-tests/wpt#21568 (we could add more tests for semantics/maction with more than one child, but I guess that's enough for now).