Add a threadpool parameter to Channel constructor (JuliaLang#50858)

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.

On 1.8, without threadpools:
```julia
% julia +1.8 -t 8
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.8.5 (2023-01-08)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end

threadid=2
threadid=5
threadid=2
threadid=2
threadid=1
threadid=6
threadid=7
threadid=8
threadid=3
threadid=4
```

On 1.9, with no interactive threads:
```julia
% julia +1.9 -t 8
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.9.2 (2023-07-05)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end

threadid=4
threadid=4
threadid=4
threadid=2
threadid=3
threadid=1
threadid=7
threadid=5
threadid=8
threadid=6
```
On 1.9, with an interactive thread:
```julia
% julia +1.9 -t 7,1
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.9.2 (2023-07-05)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
```

With this PR, the `:default` threadpool is used instead.
```julia
% julia +master -t7,1
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.11.0-DEV.244 (2023-08-09)
 _/ |\__'_|_|_|\__'_|  |  Commit d99f249* (0 days old master)
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end

threadid=7
threadid=6
threadid=7
threadid=7
threadid=6
threadid=3
threadid=5
threadid=2
threadid=4
threadid=8
```
And, the behavior can be overridden.
```julia
% julia +master -t7,1
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.11.0-DEV.244 (2023-08-09)
 _/ |\__'_|_|_|\__'_|  |  Commit d99f249* (0 days old master)
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true, threadpool=:interactive) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
```

---------

Co-authored-by: Nathan Daly <[email protected]>

Author: 2 people