feat: TEI (XML) to MinML converter

[phamelink]

feat: TEI (XML) to MinML converter

Summary

Adds a TEI/XML → MinML converter to demonstrate MinML as a viable target format for existing XML-based document standards.

• XML tree parser (go/markup/xml/treeparser.go): parses an XML stream into an AST using Go's encoding/xml, preserving element and attribute names including namespace prefixes.
• TEI → MinML conversion (go/markup/minml/cmd/minml.go): wires the tree parser into the minml CLI so users can convert a TEI XML file to MinML with a single command.

Test plan

• Run go test ./go/markup/xml/... to verify the XML tree parser and converter
• Run minml from-xml <file.xml> and confirm the output is valid MinML. A real-world TEI file to test with: EHRI-ET-DEGOB0005_HU_EN.xml

[phamelink]

Needs to be reviewed after we merge minml-lsp-v2

[AntoineBastide47]

Can you merge main into this branch ? Because the code from the minml-lsp-v2 branch is still marked as new here.