Specify grammar rules around `?[`

Cf. decision to use `e?[e]` to denote null-aware index lookup, and disambiguate by enforcing that a successful parse of a conditional expression prevents parsing a map entry.

[Edit: We do not commit to having null-aware function calls at this point. Deleted the parts about that.]

Start by reviewing the issue's decision about using `e?[e]` for null-aware index lookup and the conditional-expression/map-entry parsing interaction. Define the grammar rules and disambiguation behavior, while treating null-aware function calls as out of scope; no files or tests are named in the issue.

Written by the indexing model from the issue text.