tidewater: language-tagged value support in app

Right now the syntax for metadata fields is:

Value ::= Char*
Field ::= Value (#xFFFF Value)*

We can support language-tagging pretty easily by updating this syntax to be:

Value ::= Char*
Language ::= Char*
TaggedValue ::= Value (#xFFFE Language)?
TaggedField ::= TaggedValue (#xFFFF TaggedValue)*

Acceptance Criteria:

• tidewater understands a U+FFFE delimiter to provide language-tagging information in values and renders it using xml:lang in the to_oai_dc method.
• The tidewater documentation has been updated to reflect this.