Better error for trailing commas in `using`

Leaving a trailing comma after the last element of `using` is a mistake I've made a few times. The parser only catches this error later, resulting in a fairly poor error message. For example, it can gobble up the `function` keyword in the next part of the file:

```julia
julia> using A: b,

function foo()
end

ERROR: ParseError:
Error: extra tokens after end of expression
@ REPL[4]:3:9
using A: b,

function foo()
--------^^^^^^---
end
```

Reproduce the Julia REPL example with a trailing comma in `using A: b,` followed by `function foo()`. Start by tracing how the parser handles the `using` statement and the following `function` keyword. Done means the trailing comma produces a direct, useful parse error instead of consuming the next construct and reporting extra tokens later.

Written by the indexing model from the issue text.