Allow to substitute line numbers in diagnostic messages for cell positions

[krassowski]

A tiny nice-to-have improvement would be if the linter messages which include line numbers could have "error in line 84" substituted to "error in cell 3 line 12" using user-customizable regular expression. We could have pre-defined regular expressions as a part of language server schema so that users do not have to do it themselves. It would need to be selective to only be run in notebooks.

Maybe this would need to be only run just before the diagnostics get displayed so that we can still regexp filter on the original ones?

Edit: this could be also applied to errors returned by server (e.g. "cannot refactor: variable with this name already present in line X").