update `as_html` to accept "\n" reformatting

[ayogasekaram]

rtables/R/as_html.R

Line 85 in bdafa9d

cells <- matrix(rep(list(list()), (nlh + nrow(x)) * (nc)), ncol = nc)

initialization uses the matrix object which does not account for new lines from any text wrapping using "\n".
replace the calculation with the "string matrix"