add a {line: true} option to axes

[Fil]

handy for materializing the vertical axis of a bar chart with an ordinal x
note that in the case of a continuous X, tickX([extent min]) is often preferable, since it creates the support line for all x/y facets

see https://observablehq.com/@fil/axis-support-406

suggested by @EE2dev

[enjalot]

this makes me want to have a { frame: true } option that adds a frame (to the bottom of the marks?). For most facet use cases a default frame is probably desired and good enough.
this wouldn't replace using the frame mark to create a customized frame, just make it easier to have one when exploring.

[mbostock]

This seems like a good option to have, but I’m struggling with the name “support”—that doesn’t resonate with me. D3 calls this the “domain path”. Maybe axisLine would be okay? Should we allow you to style this line separately from the grid? It does seem related to the “frame” concept, too, so maybe frame would be okay as an option.

[Fil]

Is "support" a frenchism? Maybe a better translation for what I had in mind is “bearing” or “beam”.

Anyway, in this PR the axis line would be added (or "not removed" :)) on the materialized axes, not on all facets.

By contrast, Plot.ruleX([0]) and Plot.frame are present on all facets (more precisely "active" facets #332).

[mbostock]

Needs a better name than “support” but otherwise I’m onboard with the option.

[Fil]

axisLine is fine, I think?

[mbostock]

What about simply the name “line”? That seems to match the name “grid” pretty well, and I can’t think of any obvious ambiguity with the shorter name.

[Fil]

(example for the CHANGELOG)