Allow index node to be parsed from enumerable keys

[ssalbdivad]

Currently, an index signature including a literal key like "a" is an error due to some limitations of how the parser is implemented. There is a normalizeIndexKey function, but it is not used when parsing a schema.

If it were integrated somehow, that logic could be removed from the primary arktype package in parseObjectLiteral and it would generally be better to reduce a type like this than throw.