New syntax error on 1.8+

On 1.8:

```jl
julia> A[a = a, b...]
ERROR: syntax: invalid syntax a = a
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1
```

Not sure if this is intended. This syntax was valid on 1.7. Affects code in the wild, for example:

https://github.com/ITensor/ITensors.jl/blob/cbbe909dcbda4cf70c8ecb7679128962cda3ebc4/src/mps/abstractmps.jl#L1419-L1421