• refactor(xml-helpers): use ES6 method syntax

• refactor(xml-helpers): document & check function signature types

see: https://www.typescriptlang.org/docs/handbook/intro-to-js-ts.html

• refactor(xml-helpers): satisfy TS type checks

• et.ElementText is not always a string but always has #toString

• et.ElementTag is not always a string, but converting it to one should be sufficient for our purposes
• et.Element is not a constructor, so it should be called w/out new