Fix  querySelectorAll wrapper to correct List structure

The list object that `jsQuerySelectorAll` returned did not follow
correct structure to work with FFI. Fixed by following jsGetChildren's
implementation.

Signed-off-by: Rohan Jain <crodjer@gmail.com>
examples/list: Demonstrate `querySelector`

Uses the new `elemByQuerySelector` and `withQuerySelectorElems`
functions.

Signed-off-by: Rohan Jain <crodjer@gmail.com>
Merge pull request #235 from crodjer/query-selector-ffi

Query selector `querySelectorAll` fixes and example