Improve `<ec:InclusiveNamespaces PrefixList="...">` support

[kou]

<ec:InclusiveNamespaces PrefixList="..."> has target prefixes as space separated list:

https://datatracker.ietf.org/doc/html/rfc3741#section-4

The list is in NMTOKENS format (a white space separated list).

If XMLua provide a convenient feature to use the PrefixList value, it'll be convenient.

See also: #37 (comment)