stricter LtxParser

[sonnyp]

ltx.parse('<o/>f')
ltx.parse('<o></o>f<b/>')
ltx.parse('<o>f</b>')

should fail, currently extra data is ignored

[sonnyp]

xmpp.js does something like this

https://github.com/xmppjs/xmpp.js/blob/main/packages/xml/lib/Parser.js

Not sure how much ltx should enforce strict xml or what should happen on invalid xml. Perhaps it's best to leave to the users to decide?

In which case this should be documented eventually.