Expose warnings

[tophtucker]

Currently, ⚠️ warnings are just sent to console.warn. It'd be great if they were also exposed programmatically, so other code (like Observable itself) can consume them.

Possible approaches:

• as a property on the DOM node, like plot.scales
• as an onWarn callback
• …? add your own!

It'd be great if the warning came with information about where it originated, as if we could place an error caret at the spot in the configuration object where you should look: somewhere in the nth mark, in the x scale, etc.

Related links (mostly a note to self):

• Warning / logging system? #536
• More warnings #755
• https://github.com/observablehq/plot/blob/1df1d85818f082779d902d1588326880dc332e6c/src/warnings.ts