Add substitution

[bdarcus]

This is the default config, and by far the most common configuration we see in existing 1.0 styles:

  substitute:
    author:
      - editor
      - translator
      - title # this still needs work; it assumes no formatting

csl-next/src/style/options.ts

Line 19 in e0965d8

export type Substitute = "editor" | "translator" | "title";

It should be straightforward enough: modify formatContributors to do the substitution, and then keep track of having substituted (in the ProcHints?).