[Feature] Add custom expect messages

[Mr-Pepe]

I was looking for a way to specify custom error messages for assertions and it looks like they are available in Node.js Playwright. How much effort would it be to also implement them in Python? With a little hint on where to start, I could give it a try.

[alexspurling]

I would also like this feature. At the very least let's update the documentation so that it explicitly states this is a feature that is not available (and perhaps even with a link to this Github issue so those who want it can voice their support).

[aspenboy]

+1 for this feature!

[at0m1x19]

+1 would be really nice to have it

[mgolubovich]

+1 really need this one!