ext-mbstring replaced with polyfill

[vanodevium]

Just idea. But I think it's useful

as you can see, I have some problems with pipelines. It looks like problem with gh rate limit, so it's not a problem with source code

[jawira]

First of all thanks for your commit, sorry to have been slow to respond.
I like the idea of using polyfill, but I don't why you are handling MB_* constants since the polyfill is supposed to handle these.
Do you think is possible to only change "composer.json" ?

[vanodevium]

@jawira Hi! Because these constants are really different in old and new versions of php.
I only unified behavior of tests with different versions.

[jawira]

Hi, I really appreciate your time and effort, but I cannot merge this PR since poly-fill doesn't handle simple/full case-mapping. Also tests don't pass.
Thank you :)