R1:45718268a96e

Restore viewer() function to "Responsible Users" tokenizer in Differential

Summary:
Ref T10939. This makes the `viewer()` function work again. It retains its own meaning (viewer, plus all their projects and packages).

There's no `exact-viewer()` function; we could conceivably add one eventually if we need it.

Test Plan:
- Queried for `viewer()`, got the same results as querying by my own username.
- Browsed function in token browser.
- Reviewed autogenerated documentation.

Reviewers: chad…