Inputs.file is not a function

[mbostock]

Due to the caveat described here:

https://github.com/observablehq/inputs#inputsfileoptions

Related #29.

[mbostock]

This is much easier to solve with #294!

[mbostock]

More specifically: the new @observablehq/stdlib does not depend on @observablehq/inputs because the concept of recommend libraries has been pulled out of the standard library. And hence we can change @observablehq/inputs to depend on @observablehq/stdlib and just have it work. But this will be a breaking change along with the necessary changes to bundling and stylesheets described in #29.