BigInteger coming to JavaScript CodeGen

Support for BigInteger type in SmartPascal is being added in the DWScript repository, it is mapped to the new BigInt JavaScript type which is now supported by the major browsers.

This is going to be a work in progress over a few weeks, in part because the JS implementation is rather bare-bones with few support functions, and in part because with the BigInt type, the JS designers did away with some automated and implicit casting (uh?).