Expose tag name encoding

[josh-hemphill]

Is your feature request related to a problem? Please describe.
When implementing your own element parser, all XML illegal characters have to be manually escaped.

Describe the solution you'd like
Some way to reimplement this behavior in custom parser functions.

Describe alternatives you've considered
Importing the dom or utils library that xmlbuilder2 uses, but I was not able to find what function xmlbuilder2 is using internally.

[oozcitak]

If you are asking about the character production in the XML spec, I can expose the internal function to the parser.

Edit: Here is the function: https://github.com/oozcitak/xmlbuilder2/blob/master/src/builder/dom.ts#L34