Overview

Fixes #5983

This PR fixes the issue where docs would fail to parse with a terrible empty error message when they contained an eval block that ended with a cases, such as

x.doc = {{
  ```
  cases x -> 1
  ```
}}

(or just try editing all @unison/cloud docs for many more realistic examples in the wild).

It took a bit to track down, but the issue is that we need to generate a random name for the variable binding, when desugaring to a lambda, for which we use a unique name generator that is a function of next token's position in the file. If there are no more tokens in the file, it fails. (Tokens in an eval block are lex'd separately, so we hit EOF at the end of each one).

I fixed this by simply allowing unique name generation at the end of a file, by using the dummy position Pos (-1) (-1).

Test coverage

I added a regression test