Add default rule width setting

[lvjr]

We need to add a new option rulewidth for setting default width for all hlines and vlines. It corresponds to \arrayrulewidth in tabular.

[lvjr]

Oh, there is already a \rulewidth variable used in source code.

[lvjr]

Maybe the interface could be

rules = {fg=brown, wd=1pt}

See also issue https://github.com/lvjr/tabularray/issues/172.