Add exponentFormat support to contour lines

[eMaringolo]

I can set the exponent format to the axis, but there is no similar setting for the contour lines, so I end up with different formats for the contour lines and the axis.

Example: https://codepen.io/emaringolo/pen/vYmovwW

ps: I asked this in the community forum before, but there was no response, and I didn't find it by looking at the code: https://community.plotly.com/t/exponent-format-in-contour-lines/55545