Consider to simplify indexed offsets syntax

[mikir]

Currently, the indexed offsets have the following syntax:

struct Schema
{
    offset myOffsetArray[];
    uint8 someValue;
myOffsetArray[@index]:
    string stringArray[];
};

Because offset expressions will be disabled (#641), we can think to introduce simplified indexed offsets:

struct Schema
{
    offset myOffsetArray[];
    uint8 someValue;
myOffsetArray[]:
    string stringArray[];
};

However, the old syntax should be still valid because of the backward compatibility.

For more info, please have a look to the issue https://github.com/ndsev/zserio/issues/640.