Add a threadpool=:default parameter to Channel constructor

Without this, the task created by a `Channel` will run in the
threadpool of the creating task; in the REPL, this could be the
interactive threadpool. With this, the `:default` threadpool is
used instead, and that behavior can be overridden.

base/channels.jl

@@ -59,7 +59,7 @@ Channel(sz=0) = Channel{Any}(sz)
 
 # special constructors
 """
-    Channel{T=Any}(func::Function, size=0; taskref=nothing, spawn=false)
+    Channel{T=Any}(func::Function, size=0; taskref=nothing, spawn=false, threadpool=:default)
 
 Create a new task from `func`, bind it to a new channel of type
 `T` and size `size`, and schedule the task, all in a single call.
@@ -70,9 +70,12 @@ The channel is automatically closed when the task terminates.
 If you need a reference to the created task, pass a `Ref{Task}` object via
 the keyword argument `taskref`.
 
-If `spawn = true`, the Task created for `func` may be scheduled on another thread
+If `spawn=true`, the `Task` created for `func` may be scheduled on another thread
 in parallel, equivalent to creating a task via [`Threads.@spawn`](@ref).
 
+The `threadpool` argument is used if `spawn=true`, to spawn the new Task to the
+specified threadpool.
+
 Return a `Channel`.
 
 # Examples
@@ -117,6 +120,9 @@ true
     In earlier versions of Julia, Channel used keyword arguments to set `size` and `T`, but
     those constructors are deprecated.
 
+!!! compat "Julia 1.9"
+    The `threadpool=` parameter was added in Julia 1.9.
+
 ```jldoctest
 julia> chnl = Channel{Char}(1, spawn=true) do ch
            for c in "hello world"
@@ -129,12 +135,13 @@ julia> String(collect(chnl))
 "hello world"
 ```
 """
-function Channel{T}(func::Function, size=0; taskref=nothing, spawn=false) where T
+function Channel{T}(func::Function, size=0; taskref=nothing, spawn=false, threadpool=:default) where T
     chnl = Channel{T}(size)
     task = Task(() -> func(chnl))
     task.sticky = !spawn
     bind(chnl, task)
     if spawn
+        Base.Threads._spawn_set_thrpool(task, threadpool)
         schedule(task) # start it on (potentially) another thread
     else
         yield(task) # immediately start it, yielding the current thread