Try to fix struct revision

[IanButterworth]

With JuliaLang/julia#57253 merged structs are now modifiable, which works in the repl, but doesn't appear to in Revise.

Here just modifying a struct in Example to add a new field.

julia> using Revise, Example

julia> Example.Foo(1)
Example.Foo(1)

julia> Example.Foo(1,2)
ERROR: MethodError: no method matching Example.Foo(::Int64, ::Int64)
The type `Example.Foo` exists, but no method is defined for this combination of argument types when trying to construct it.
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

This PR is a trial-and-error attempt to figure it out. I'm opening it mainly as breadcrumbs incase this is helpful for figuring out the real fix.

With this the eval fails,

julia> Example.Foo(1)
evaluating _J1 = ($(QuoteNode(Core._structtype)))(Example, :Foo, %J3, %J4, %J5, false, 2)
┌ Error: Failed to revise /Users/ian/.julia/dev/Example/src/Example.jl
│   exception =
│    syntax: invalid assignment location "JuliaInterpreter.SlotNumber(id=1)"
│    Stacktrace:
│     [1] top-level scope
│       @ ~/.julia/dev/Example/src/Example.jl:4
│    Revise evaluation error at /Users/ian/.julia/dev/Example/src/Example.jl:4
│
└ @ Revise ~/Documents/GitHub/Revise.jl/src/packagedef.jl:776

[IanButterworth]

#894 is the real deal