DAST and XML Compatability

[siefkenj]

This PR

• updates the Lezer grammar to support all XML features, so valid XML documents should parse as DoenetML documents.
• Introduces DAST (DoenetML Abstract Syntax Tree) as a format for interacting with DoenetML source code. The old parser.ts is still around, but it should be reworked to use DAST.
  • DAST has a type: "error" node that contains information about parse errors.
  • the toXml function can output proper XML or DoenetML-style markup (e.g., with JSX-style properties). It can also output explicit <_error> nodes where errors were encountered, or it can silently omit them.
• A basic pretty-printer based off of Prettier. Lots more work needs to be done on the pretty printer to make sure it satisfies all the edge cases.
• Add a showFormatter attribute to CodeEditor that will allow for pretty printing of code and for converting between XML and DoenetML

[dqnykamp]

+1