Add support for projections to the standalone Plot.scale function

[Fil]

e.g. Plot.scale({projection: {type: "mercator", width, domain: …}}).

When merged, we must add this to the changelog PR #2426:

The standalone [Plot.scale](https://observablehq.com/plot/features/scales#scale) function now also supports projections.

```js
const projection = Plot.scale({projection: {type: "mercator"}});
projection.apply([-1.55, 47.22]) // [316.7, 224.2]

const plot = Plot.plot({projection, marks: [Plot.sphere()]});
```

[mbostock]

Suggested change

	Plot.scale also supports projections. <VersionBadge version="0.6.18" /> The returned projection object exposes *apply* and *invert* methods for converting between geographic and pixel coordinates, and can be passed as the **projection** option of another plot.
	Plot.scale also supports projections. <VersionBadge pr="2427" /> The returned projection object exposes *apply* and *invert* methods for converting between geographic and pixel coordinates, and can be passed as the **projection** option of another plot.

Let’s not get ahead of ourselves. 🙂

[mbostock]

Is options really optional here?

Suggested change

	export function scale(options?: {[name in ScaleName]?: ScaleOptions}): Scale;
	export function scale(options: {[name in ScaleName]?: ScaleOptions}): Scale;

[mbostock]

Should this be called exposeProjection for symmetry?

[Fil]

Yes. There is already an exposeProjection function but it could be renamed to prepareProjection.

[mbostock]

This doesn’t look like what we do in autoHeight…

[Fil]

True. I'll make a helper function to be used in both places.

[mbostock]

As far as testing goes, I’m hoping that we can test this new implementation against the existing (more wasteful) one. This:

Plot.plot({projection: {type: "mercator"}}).scale("projection")

Should be the same as this:

Plot.scale({projection: {type: "mercator"}})

[mbostock]

This looks like it needs margin options, too. And probably aspectRatio? I guess those should be pulled out of PlotOptions and into something we can reuse here, maybe call it DimensionOptions?