[Feature] expect.soft() for Python

[mentalfabric]

Sure would be nice if we could get soft assertions on the Python side. We're currently trying implementing Playwright for our test automation at my workplace and any time an expect fails it ends the entire test. This means that we have to revert back to using the Pytest with check asserts whenever we just need a soft assertion.

Please implement expect.soft() on the Python side of Playwright. It already exists for the Javascript side.

[mxschmitt]

Folding into #1272