errorshow: simplify printing of keyword argument types using a new macro format

[aviatesk]

In Julia, keyword arguments are represented as Base.Pairs objects. However, the object type often appears unnecessarily complex, especially when printed in a stack trace.

This commit aims to simplify the printing of stack traces that involve keyword method calls, while still allowing us to reconstruct the actual method signature types from the printed signature types.

The approach is similar to #49117: this commit introduces a new macro called Base.@Kwargs. It follows the same syntax as @NamedTuple and returns a Base.Pairs type that is used for keyword method calls. We use this syntax when printing keyword argument types.

Here's an example of a stack trace for sum("julia"; init=1):

diff --git a/b.jl b/a.jl
index 91dd6f0464..b804ae4be5 100644
--- a/b.jl
+++ b/a.jl
@@ -22,12 +22,11 @@ Stacktrace:
     @ Base ./reduce.jl:44 [inlined]
   [6] mapfoldl(f::typeof(identity), op::typeof(Base.add_sum), itr::String; init::Int64)
     @ Base ./reduce.jl:175 [inlined]
-  [7] mapreduce(f::typeof(identity), op::typeof(Base.add_sum), itr::String; kw::Base.Pairs{…})
+  [7] mapreduce(f::typeof(identity), op::typeof(Base.add_sum), itr::String; kw::@Kwargs{init::Int64})
     @ Base ./reduce.jl:307 [inlined]
-  [8] sum(f::typeof(identity), a::String; kw::Base.Pairs{Symbol, Int64, Tuple{Symbol}, @NamedTuple{init::Int64}})
+  [8] sum(f::typeof(identity), a::String; kw::@Kwargs{init::Int64})
     @ Base ./reduce.jl:535 [inlined]
-  [9] sum(a::String; kw::Base.Pairs{Symbol, Int64, Tuple{Symbol}, @NamedTuple{init::Int64}})
+  [9] sum(a::String; kw::@Kwargs{init::Int64})
     @ Base ./reduce.jl:564 [inlined]
  [10] top-level scope

Feel free to share any comments or suggestions. If this idea seems acceptable, I will add test cases and also address any broken test cases.

[KristofferC]

If it is exported then the Base. prefix in Base.@Kwargs{init::Int64} would disappear. That seems worth it to me.

[aviatesk]

I didn't export it because I considered it as an internal implementation detail that keyword arguments are represented as Base.Pairs. But I'm fine with exporting it as I agree with the benefit of further simplifying the stacktrace.

[KristofferC]

I didn't export it because I considered it as an internal implementation detail that keyword arguments are represented as Base.Pairs.

If that implementation changes then @Kwargs would also change. I don't think a user of @Kwargs can expect that the return type is fixed, only that it returns the same one as used for keyword arguments.

[aviatesk]

I enabled the printing with the @Kwargs-macro syntax only within stack trace view. Please let me know if you think it is more reasonable to enable the printing in the other situations.

The main reasoning behind the current choice is that Base.Pairs is a general data structure and its usage is not restricted to represent keyword arguments.

[aviatesk]

I'd argue that 6 👍 means this PR is preferable. I'm planning to merge this PR tomorrow if there are no objections.