Extensible help system for displaying MethodErrors

[mbauman]

Fixes #7512. This adds a hook for developers to add custom help messages when
MethodErrors get displayed. Feel free to bikeshed the naming of this method.

This is particularly helpful in cases of intentionally unimplemented methods.
I've also restructured the existing help messages in terms of this new system,
using the info command to display them (and a few extra messages, too):

julia> Int("1")
ERROR: MethodError: no method matching convert(::Type{Int64}, ::String)
INFO: Cannot `convert` an object of type String to an object of type Int64.
INFO: This may have arisen from a call to the constructor Int64(...), since type constructors fall back to convert methods.

Stacktrace:
 [1] Int64(::String) at ./sysimg.jl:114

julia> min([1,2,3])
ERROR: MethodError: no method matching min(::Array{Int64,1})
INFO: Use minimum() to find the smallest element of an array.

Closest candidates are:
  min(::AbstractArray{T1<:Real,N} where N, ::Real) where T1<:Real at deprecated.jl:55
  min(::AbstractArray{T1<:Real,N} where N, ::AbstractArray{T2<:Real,N} where N) where {T1<:Real, T2<:Real} at deprecated.jl:55
  min(::Any, ::Any) at operators.jl:404
  ...

julia> [1,2,3](1)
ERROR: MethodError: no method matching (::Array{Int64,1})(::Int64)
INFO: Use square brackets [] for indexing into arrays.

julia> [x for x in nothing]
ERROR: MethodError: no method matching start(::Void)
INFO: Objects of type Void are not iterable.

Closest candidates are:
  start(::SimpleVector) at essentials.jl:534
  start(::Base.MethodList) at reflection.jl:659
  start(::ExponentialBackOff) at error.jl:147
  ...
Stacktrace:
 [1] collect(::Base.Generator{Void,getfield(Main, Symbol("##1#2"))}) at ./array.jl:656

Feel free to bikeshed the name (no_method_error_help).

This is an old branch/idea that I had totally forgotten about; I was reminded by #24284, which would be very trivial in this setup.

[mauro3]

This is great! Bike-shedding the name, maybe methoderror_help to mirror MethodError. Also, I think this printing makes equally sense in non-interactive use.

Interface-wise it would be nicer if instead of

no_method_error_help(io, ::AbstractArray, args...) = info(io, "Use square brackets [] for indexing into arrays.")

this was possible:

(::AbstractArray)(args...) = throw(MethodError( "Use square brackets [] for indexing into arrays."))

whilst still retaining method_exists(AbstractArray, (Vararg,))==false. (Although that also runs into #14919). However, I suspect that this is much harder, let's go with this.

[mbauman]

Implemented in #35094