stam fromxml: expand yet simplify configuration language, allow full templating

[proycon]

The current configuration language for stam fromxml can be improved to be more powerful by introducing templating more consistently, at the same time this can also simplify the language.

Things I want to implement:

• we already have {@x} for variables currently, but that's about it. We need blocks like {% if @x == "blah" %}{@x}{% else %}{}{% endif %}
• allow XML namespaces for variables in templates, e.g. {@xml:id}
• allow accessing parent elements {../@xml:id}
• allow accessing child elements {./child/@xml:id}
• have a syntax that raises an error if an attribute doesn't exist.
  • This is the default now unless the attribute is predeceded by ?. or skip_if_missing is set on the annotation data
• conditional selects: /path/to/element[@x == "blah"] (without implementing all of XPath)

It's probably easiest to use an existing templating engine like upon, liquid or tera.

[proycon]

First implementation is merged, not entirely done yet, also waiting on a 3rd party dependency release. Initial documentation for the language is here: https://github.com/annotation/stam-tools/blob/master/docs/fromxml.md

Feedback appreciated!

[proycon]

I tried the new stam fromxml on the Israel's corpus, see HuygensING/israels#1 and the included tei.toml there.