more Unicode dots as infix operators

[tpapp]

This is a feature request to add … (\dots) and ⋯ (\cdots) as infix operators, possibly with the same precedence as .. (8).

Currently, … cannot be used as a function name at all, while ⋯ works, but is not infix:

julia> …(a,b) = a+b
ERROR: syntax: invalid character "…"

julia> ⋯(a,b) = a+b
⋯ (generic function with 1 method)

julia> 1⋯2
ERROR: UndefVarError: ⋯2 not defined

julia> ⋯(1,2)
3

This would be useful to define alternatives to : (with different, but related semantics) in packages. @timholy's IntervalSets.jl uses .., other operators would be good to have to avoid conflict.

(Incidentally, font rendering of these characters was discussed in #8636, but as they are supported already I assume this is not an issue).

[JeffBezanson]

Looks like we should add \cdots to the .. precedence level at least. I'm less sure about \dots, and that would be a less breaking change since it's currently disallowed.