Add string validation to isJsonString assertion

[EmilMassey]

Assertion::isJsonString returns true if the value is not a string but is an accepted argument for the json_decode function. This is possible because the library works in non-strict mode, so PHP does not raise an error if a non-string argument is passed to json_decode.

Current behavior

None of these throw an exception:

Assert::that(true)->isJsonString();
Assert::that(10)->isJsonString();
Assert::that(2.5)->isJsonString();

Expected behavior

All of the above should throw AssertionFailedException

Psalm

Because the function is annotated with @psalm-assert =string $value, Psalm does not raise an issue with this snippet:

declare(strict_types=1);

function test(mixed $value): string {
    Assertion::isJsonString($value);

    return $value;
}

echo test(5);

This will actually fail at runtime with Fatal error: Uncaught TypeError: test(): Return value must be of type string, int returned. My PR changes this, so AssertionFailedException is thrown instead.

Backward compatibility

This will be a BC break if someone used the assertion with non-string arguments. On the other hand, description clearly suggests that this assertion should be performed for strings.