add array support for metadata, fixes #4

[tacman]

I updated the docs, attribute and tests.

It got me thinking, though, if null should be allowed as a valid metadata value.

Nonetheless, I think this fixes #4

[tacman]

I fixed the phpstan errors, except for one:

vendor/bin/phpstan

::error file=src/Metadata.php,line=22,col=0::Method Zenstruck\Metadata::__construct() has parameter $value with no value type specified in iterable type array.

final class Metadata
{
    public function __construct(
        /** @readonly */
        public string $key,

        /**
         * @readonly
         * @param string|bool|int|float|mixed value
         */
        public string|bool|int|float|array $value,
    ) {
    }

What should this be?

[kbond]

Looks good! I think my suggestions will fix phpstan.

It got me thinking, though, if null should be allowed as a valid metadata value.

Couldn't hurt but maybe wait until someone needs it.

[kbond]

I'll take a look at fixing phpstan before merging.

[kbond]

Thanks @tacman!