Allow calling readField with ReadFieldOptions.

[benjamn]

Note: I decided to split these changes out from PR #6288 after all, per my
comment #6288 (comment).

As with Cache.EvictOptions in #6288, named options also make sense for the options.readField function that we pass to read and merge functions.

While it was/is technically possible to specify field arguments by passing a FieldNode instead of a string as the first argument to readField (as I was recently reminded while investigating #6278), I think it makes a whole lot more sense to let readField accept named options, so that we can more easily add fields like args (and other optional fields, in the future):

new InMemoryCache({
  typePolicies: {
    Query: {
      fields: {
        messages(_, { args, toReference, readField }) {
          return readField({
            from: toReference({
              __typename: "Chat",
              id: args.id,
            }),
            fieldName: "messages",
            args: { offset: 0, limit: 10 },
          });
        },
      },
    },
  },
})

As with cache.evict, options.readField can still be called with just a string, or a string and a foreign StoreObject | Reference object, in cases when that information is sufficient. Once you get into more advanced territory, passing named options is a great way to limit code complexity.

[benjamn]

The new FieldSpecifier type was important to clean up this awkward/subtle dependency between nameOrField and argsOrVars.

[benjamn]

Merged despite the bundlesize near-miss, because #6289 should bring the bundle size down again.