Implement LST appends in the BinaryReader and BinaryWriter

[desaikd]

It seems like read support for LST appends isn't implemented in ion-js. When trying to read through LST appends, the binaryReader throws an error stating Can't step in to a scalar value while reading symbol table declarations. LST declarations use an array of structs to define all the symbol tables they’d like to import but while reading an LST append, the imports field's value is symbol $ion_symbol_table, indicating the use of existing table. So the possible cause is that ion-js doesn't recognize this format.