examples: add xml parser, add parse.sh scripts

[milahu]

parse.sh is useful for prototyping new parsers

also needed because nix repl caches imports, so i cannot use a devloop like

((import ./examples/uuids/default.nix {})).parseUuid "123e4567-12d3-a456-426614174000"

also

• add xml parser to examples/xml
  • parse-xml.nix
  • parse-xml.test.js - test runner
  • parse-xml.test.md - test snapshots
  • parse-xml-comment.nix - demo parser with lookahead, based on function takeUntil
  • print-xml.nix - serialize parse tree to xml
  • print-yaml.nix - serialize parse tree to yaml

todo

• [ ] use examples/xml/parse-xml.sh as template for all parse.sh
• [ ] add markdown parser
• [ ] write test runner in Nix