Support Julia v1.5 short keyword argument syntax

[ericphanson]

julia> using Mocking

julia> g(; x=1) = "I got x=$x"
g (generic function with 1 method)

julia> function f()
           x = 1
           result = @mock g(; x)
           return result
       end
ERROR: LoadError: MethodError: Cannot `convert` an object of type Symbol to an object of type Expr
Closest candidates are:
  convert(::Type{Expr}, ::Revise.RelocatableExpr) at /Users/eph/.julia/packages/Revise/9lZUE/src/relocatable_exprs.jl:22
  convert(::Type{T}, ::T) where T at essentials.jl:218
  Expr(::Any...) at boot.jl:263
Stacktrace:
 [1] push!(a::Vector{Expr}, item::Symbol)
   @ Base ./array.jl:968
 [2] extract_kwargs(expr::Expr)
   @ Mocking ~/.julia/packages/Mocking/U41JO/src/expr.jl:22
 [3] var"@mock"(__source__::LineNumberNode, __module__::Module, expr::Any)
   @ Mocking ~/.julia/packages/Mocking/U41JO/src/mock.jl:10
in expression starting at REPL[15]:3

(This works of course if we write @mock g(; x=x)).