[PHP 8.2] Fix `${var}` string interpolation deprecation
[PHP 8.2] Fix `${var}` string interpolation deprecation

PHP 8.2 deprecates `"${var}"` string interpolation pattern.
This fixes all 8 of such occurrences in `voku/portable-utf8` package.

 - [PHP 8.2: `${var}` string interpolation deprecated](https://php.watch/versions/8.2/${var}-string-interpolation-deprecated)
 - [RFC](https://wiki.php.net/rfc/deprecate_dollar_brace_string_interpolation)

voku/portable-utf8