[Syntax evolution] Disable mixing of keyword arugments with positional arguments

[MasonProtter]

We currently allow some really weird stuff with keyword arguments going before or even between positional arguments:

julia> f(a; b, c) = a + b + c
f (generic function with 1 method)

julia> c = 3
3

julia> f(1, b=2; c)
6

julia> f(b=2, 1; c)
6

I think that this was likely a mistake, and now that we have synatx evolution, we should probably just make this a syntax error.

[Keno]

Go for it (speaking in my personal capacity :) )!

[MasonProtter]

One comment from @StefanKarpinski on Slack is that he likes being able to put kwargs in with positional args, but he does say that he

would be happy to enforce that if you use a semicolon in a function call then everything before it must be positional and everything after it must be keyword