fix bcmath types.

[Warxcell]

Reference in psalm: https://psalm.dev/r/3786d05a6c

[ondrejmirtes]

I'm hesitating to merge this for same reason as here: #2163 (comment)

With this update, users need to update their PHPDocs with numeric-string...

[orklah]

If it can help with deciding, bcadd(and others) is documented as taking numeric-strings in Psalm for almost 3 years now: vimeo/psalm#3189

As far as I know, no one complained about that. Though I admit it may be biased because users likely to come and create an issue are users that generally will want more strict checks,

[Warxcell]

I'm hesitating to merge this for same reason as here: #2163 (comment)

With this update, users need to update their PHPDocs with numeric-string...

but that's a good thing, right? I mean: if they accept user string - the method that returns that string will be just string - so phpstan will force them to take that string through is_numeric check, otherwise they will get an exception in runtime - which is basically what static analysers are trying to prevent.

[ondrejmirtes]

Commited as 3fb919a