I am not a programmer but I cannot see how that is currently possible using Joplin as everything that is enclosed in the fenced code block is apparently not controlled by Joplin, but by the highlighter package Joplin uses.
The only workaround I can think of is to not use fenced code, manually add the line numbers and tab indent each line.
Of course you lose code highlighting using this method and for lengthy code listings it could be quite time-consuming.