minimal support for bigint

[Fil]

closes #1278

@llimllib see if you can break this branch by passing bigints to unsuspected places (a build is available at https://observablehq.com/@observablehq/plot-bigint-1278 )… note that we're supporting bigints only as channel values, not as options (e.g. we don't try to support inset: 1n or margin: 10n).

As I was researching all the places where we coerce values to a number, I realized that this line is probably redundant, as we enter inferLogDomain only with already-coerced channels:

plot/src/scales/quantitative.js

Line 293 in d8378f0

v = +v;

[llimllib]

I'll try to take a hack at it this afternoon 👍

[mbostock]

I suspect there are some further tweaks we can make to coercion but this seems like a good incremental step forward!