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]

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"}})

[Fil]

note that the terse definitions here are in fact only for Plot.scale({projection}), and get overridden by more detailed definitions in Plot.plot(), i.e.

plot/src/plot.d.ts

Lines 11 to 23 in 1c6b239

	/**
	* The outer width of the plot in pixels, including margins. Defaults to 640.
	* On Observable, this can be set to the built-in [width][1] for full-width
	* responsive plots. Note: the default style has a max-width of 100%; the plot
	* will automatically shrink to fit even when a fixed width is specified.
	*
	* [1]: https://github.com/observablehq/stdlib/blob/main/README.md#width
	*/
	width?: number;
	/**
	* The outer height of the plot in pixels, including margins. The default
	* depends on the plot’s scales, and the plot’s width if an aspectRatio is