This reverts commit cbe67b9602.
Word sets `w:firstColumn="1"` by default for tables. You have to find
the Table Design tab and explicitly uncheck "First Column" to make this
go away. In most cases, I don't think writers intend to designate
the first column as a row head, so this commit is going to produce
unexpected results. In addition, because of the table normalization
done by pandoc-type's `tableWith`, any table containing a colspanned
cell in the left-hand column will get broken if the first column is
designated a row head. For these reasons it seems best to revert this
change, which was made in response to #9495.
Closes#10627.
Reader: When `w:tblLook` has `w:firstColumn` set (or an equivalent bit
mask), we set row heads = 1 in the AST.
Writer: set `w:firstColumn` in `w:tblLook` when there are row
heads. (Word only allows one, so this is triggered by any number
of row heads > 0.)
Closes#9495.
John MacFarlane