Although hopefully this can eventually be added to nodePackages, it uses
some devDependencies to build custom fonts. Node2nix doesn't currently
support enabling devDependencies for a single package.

- Got rid of redundant let-in statements.
- node-packages.json now only pulls in Iosevka.
- generate.sh
  * Uses a nix-shell shebang to ensure it builds using the current
    version of node2nix (the old version caused some issues due to the
    19.03 release version being 1.6.0 instead of 1.7.0).
  * Builds in development mode to fix the devDependencies issue.
- Use the tree of the built node package as sourceRoot instead of
  installing node dependencies manually. This means the source will have
  to be updated in both node-packages.json and default.nix, but to make
  things easier the derivation inherits the version number.
- Disparate build options now all live under privateBuildPlan, which is
  converted first with builtins.toJSON and then to TOML using remarshal
  (Unfortunately there is not currently a builtins.toTOML).
- Extra parameters can also be provided that will be converted to JSON
  then TOML. This will overwrite the default parameters.toml file.

iosevka: Simplify custom build process

The interface to configure a custom build with Nix has changed due to changes upstream; please refer to the derivation for details.