Do not allow THIN SPACE U+2009 as an operator suffix

[tkf]

I noticed #22089 allowed a THIN SPACE U+2009 to be a suffix of the operator. I guessed this was a typo since it would be very confusing:

julia> Meta.parse(string("1 ", "*", '\U2009', " 2"))
:(1 *  2)

julia> eval(ans)
ERROR: UndefVarError: *  not defined

After this PR:

julia> expr = Meta.parse(string("1 ", "*", '\U2009', " 2"))
:(1 * 2)

julia> dump(expr)
Expr
  head: Symbol call
  args: Array{Any}((3,))
    1: Symbol *
    2: Int64 1
    3: Int64 2

julia> eval(expr)
2

[Keno]

FWIW, this PR seems right to me, but @stevengj should confirm.

[KristofferC]

I don't think this should be backported.

[StefanKarpinski]

Agree: it’s technically breaking just very unlikely to cause real problems.