Support type guards for the output of the parser

[wrnrlr]

I've added support for type guards on the output of the parser by annotating the result of the parser as a discriminating union.

This will help typescript to infer the type of the Reader when checking the SaxEventType:

for await (const [event, detail] of parser.parse(reader)) {
  if (event === SaxEventType.Attribute) {
    detail.name
  } else if (event === SaxEventType.OpenTag) {
    detail.attributes
  }
}

[justinwilaby]

Nice! This is an easy approval.

[justinwilaby]

Much better! Thank you for putting this together.