PHP 8.1 Compatibility

[SychO9]

Hello! great package!

As f PHP 8.1 readonly is a unique keyword and therefore a class/interface/trait cannot be named as such.

This PR supercedes (axypro/sourcemap#7).

[vasa-c]

@SychO9 thanx

[leowhite90]

I'm trying to install this commit with composer update after 0.1.5 but getting this error
Your requirements could not be resolved to an installable set of packages.

- Root composer.json requires axy/sourcemap dev-master#aaea981 -> satisfiable by axy/sourcemap[dev-master].
- axy/sourcemap dev-master requires axy/errors ^3.0 -> found axy/errors[dev-master, dev-php5, 0.0.1, ..., 0.0.5, 1.0.0, ..., 1.0.5, 2.0.0, 2.1.0] but it does not match the constraint.

[vasa-c]

@leowhite90 will fix it soon

[vasa-c]

@leowhite90 axy\errors 3.0.0 and axy\sourcemap 1.0.0, check, please