TypeScript: read vtable entries as uint16

[jtbandes]

According to the internals docs, vtable entries are uint16 rather than int16.

This change allows reading to work when vtable entries are larger than 2^15-1 (which already works in C++). For example, the following schema is now readable:

struct FixedArray {
  // This array is large enough that it pushes the offset of another_double above INT16_MAX
  fixed_array:[double:5555];
}

table LargeOffset {
  another_double:double;
  fixed_array:FixedArray;
}

root_type LargeOffset;

(I ran into this bug in the wild when a user showed up with a schema having ~4500 fields, mostly doubles.)