Support limited JS templates in Brioche.download and other statics

[kylewlacy]

Part of #70

This PR updates the "project analysis" phase to support a limited form of JS template expressions, which can be used by statics like Brioche.download. Templates can currently only include references to the project export from the root module, or trivial values like nested string literals. This means it's pretty limited, but it does allow for one major quality-of-life improvement: referencing the version number in a download URL. Here's a minimal example (based on the jq package):

import * as std from "std";

export const project = {
  name: "jq",
  version: "1.7.1",
};

const source = Brioche.download(
  `https://github.com/jqlang/jq/releases/download/jq-${project.version}/jq-${project.version}.tar.gz`,
)
  .unarchive("tar", "gzip")
  .peel();

// ...

If the value for project.version gets updated, then the new URL will automatically get downloaded, then the new URL and hash will get recorded in the lockfile! This should make managing updates simpler