Introduces a syntax for cross-reference links that work across language
and change behavior depending on which scope they appear in.
```markdown
@[interrupt]
:::python
@[StateGraph]
:::
:::js
@[create_react_agent]
:::
```
Can be compiled to
```markdown
# a link that changes based on global context or compile target
<div> ... </div> -> `interrupt` in global context
:::python
[StateGraph](link to python cross reference)
:::
:::js
[create_react_agent](link to js cross reference)
:::
```
TODO:
- [x] fix broken unit test
- [x] no f strings in logger (it's a sin)
- [x] remove cross-refs.txt (we'll instead start updating the cross link
map)
