Add support for tooltips

[mjbright]

Description

tooltip is a standard attribute in d2 language It does not seem to be available in py-d2.

Why

• When using svg this is extremely useful - I'm using it (via d2 binary) for my application
• Parity with d2 features

Alternatives

Well unfortunately, I experimented with d2 to prototype an application and this is one feature that was missing.

For the moment I've ended up writing a Python wrapper around the d2 binary, but I'd prefer to go with py-d2 when it's well documented and with more features - esp. tooltips.