TypedArray: fix numeric property definitions

TypedArray: fix numeric property definitions

Previously, integer-indexed exotic validation was restricted to keys marked
as strings.  Numeric atoms and the canonical string "NaN" could bypass the
check and be installed as ordinary typed-array properties.

The fix is to validate numeric atoms and canonical numeric strings after key
conversion, and to remove the obsolete source-type flag.

The numeric-key bypass was introduced in b28e50b1 (0.9.0); the incomplete
canonical-index handling dates to bf208048 (0.3.8).

This closes #1098 issue on GitHub.