Support string assertions resulting in non-empty-string

[herndlm]

Closes https://github.com/phpstan/phpstan-webmozart-assert/issues/129

Brings back what was reverted in https://github.com/phpstan/phpstan-webmozart-assert/pull/89 and steals ideas from https://github.com/phpstan/phpstan-webmozart-assert/pull/128

[herndlm]

I want to check if I can make it report if those functions are called more than once with the same args

[herndlm]

Now it works as I imagined :) I'm gonna add the same double-check-report adaption to phpstan-src with str_contains if possible.

[herndlm]

jfyi in case it was missed - I think this one should be good

[ondrejmirtes]

Really nice!

[ondrejmirtes]

Thank you.