Escape newlines in graphviz output

[saulshanabrook]

This PR adds support for rendering nodes in graphviz that contain newlines (\n).

Note that the default egglog parser is not able to create strings with newlines (i.e. the string "\n" ends up with the newline escaped during parsing), but the Python bindings are able to create them.

Previously, newlines were emitted as-is in the graphviz output, as newlines in the file, which meant they were not rendered.

I tested this PR with this code:

from egglog import *

egraph = EGraph()

@egraph.function
def f() -> String: ...

egraph.register(set_(f()).to(String("Hello\nWorld!")))
egraph.display()

Which now renders like this:

But previously was like this:

EDIT: I also added a BALIGN="left" so that the lines separated by <br /> are aligned left, instead of centered, and tested this.

[saulshanabrook]

I realized you can actually reproduce this bug in egglog itself:

(function f () String)

(set (f) "Hello
World!")