This commit changes the data type of the function parameters across various files. The
modifications are made in the 'phpstan-baseline.neon', 'AbstractCurrency.php',
'CurrencyInterface.php', and 'Handler.php' files. Certain function parameters which could have been
null are now specifically marked as available for null input. Improving the type hinting of these
functions enhances code readability, allows for better static analysis and reduces the scope for
bugs in the future.

Related: #9

As the problem can be handled, it is not worth an error.

when creating currencies, e.g. crypto currencies, the currency signs were not created correctly and
translated by the translator

feat: user currency -> the user can now have his own currency, so he can use the exchange rate to display the price in his currency.