Add outer option `transpose`

[lvjr]

With outer option transpose, we could input cell text column by column.

\begin{tblr}[transpose]{}
  a & b & c \\
  d & e & f \\
  g & h & i
\end{tblr}

is the same as

\begin{tblr}{}
  a & d & g \\
  b & e & h \\
  c & f & i
\end{tblr}