Add tests for curvenote AST / myst / html / etc conversions

[fwkoch]

This adds some declarative integration tests for curvenote schemas.

For now, inputs are defined like so:

description

+++ curvenote
type: doc
content:
  ...

+++ myst
...

+++ html
...

+++ text
...

+++ tex
...

For each format, it checks that the curvenote export matches the formatted text and the formatted text import matches the curvenote AST.

Potentially these input files could also be used to generate documentation of curvenote and/or myst functionality.

We should probably consider if there is a less-ad-hoc way to define the input files / test framework, but for now this works.

[fwkoch]

The bespoke (i.e. non YAML/JSON) test files (and corresponding test framework) aren't great, plus all these tests are now in myst-spec in better form: https://github.com/executablebooks/myst-spec/tree/main/docs/examples