[css-ruby-1] Change 'collapse' to 'merge' for ruby-merge

[r12a]

https://drafts.csswg.org/css-ruby-1/#collapsed-ruby
4.2 Sharing Annotation Space: the ruby-merge property

I think that a better name for the collapse value would be merge, ie. ruby-merge: merge; The meaning of collapse is not as obvious as merge, to my mind.

[patrickdark]

Sounds like a good idea. I'm guessing the value is modeled on border-collapse: collapse, but in that case the property name and value match.

[frivoal]

This issue got duplicated as #5004, and was resolved there. Closing.