TISTUD-6683 Gracefully handle invalid XML attribute values

- be able to handle unquoted attribute values that begin with digits in our scanner
- mark unquoted attribute values as parse errors and recover in our parser
- mark attributes with no value as parse errors and recover in our parser

I'm more interested to have valid nodes after invalid nodes (or couple of invalid nodes) - to make sure our parser catches both the invalid nodes instead of failing at just the 1st node.