Replace mbstring with polyfill

[norberttech]

Currently flow (few packages) requires mbstring extension in order to use mb_strlen and few other functions.
Instead of expecting this extension to be installed we should be able to replace it with https://packagist.org/packages/symfony/polyfill-mbstring

[stloyd]

Are we sure about this? That extension seems not to be used right now anywhere in Flow:

stloyd@MacBookPro flow % composer why symfony/polyfill-mbstring
symfony/console          v7.4.4  requires symfony/polyfill-mbstring (~1.0) 
symfony/filesystem       v7.4.0  requires symfony/polyfill-mbstring (~1.8) 
symfony/framework-bundle v7.4.5  requires symfony/polyfill-mbstring (~1.0) 
symfony/http-foundation  v7.4.5  requires symfony/polyfill-mbstring (^1.1) 
symfony/string           v7.4.4  requires symfony/polyfill-mbstring (~1.0) 
symfony/translation      v7.4.4  requires symfony/polyfill-mbstring (~1.0) 
symfony/var-dumper       v7.4.4  requires symfony/polyfill-mbstring (~1.0) 
twig/twig                v3.23.0 requires symfony/polyfill-mbstring (^1.3)  

And:

stloyd@MacBookPro flow % composer why ext-mbstring             
Package "ext-mbstring *" found in version "8.3.30".
There is no installed package depending on "ext-mbstring"

[stloyd]

Never mind, seems we need to add this as an official requirement, cause we use code without announcing that extension is needed:

[stloyd]

@norberttech I'm just not sure we should put it as require-dev or main require.

For example, we have two options:

1. Do just the polyfill requirement:

    "require": {
        "php": "~8.3.0 || ~8.4.0 || ~8.5.0",
        "flow-php/etl": "self.version"
        "symfony/polyfill-mbstring": "^1.33"
    },

2. Do double requirement, for extension and polyfill:

    "require": {
        "php": "~8.3.0 || ~8.4.0 || ~8.5.0",
        "ext-mbstring": "*",
        "flow-php/etl": "self.version"
    },
    "require-dev": {
        "symfony/polyfill-mbstring": "^1.33"
    },

In the first version, we use a polyfill, always assuming that the extension may not be there, and we want to be sure we have functions for it.
In the second, we're pointing out that an extension is needed, and for development, we add a polyfill for it.

[norberttech]

I think that whatever package needs it should only have mbstring polyfill in required section - that's the easiest