Add StartImmediate to MailboxProcessor

[bmitc]

F# language suggestion: fsharp/fslang-suggestions#997
Original PR by @xloggr: #11515

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This PR implements both a static and instance StartImmediate method to MailboxProcessor to start a MailboxProcessor on the current thread. Normally, Start will start the MailboxProcessor on the thread pool, but the StartImmediate method ensures that the MailboxProcessor starts on the same thread as the one that calls it. Effectively, MailboxProcessor.Start and MailboxProcessor.StartImmediate map to Async.Start and Async.StartImmediate, respectively.

This is useful for situations in which you want to force the MailboxProcessor to start on a given thread or even run on a single thread throughout its lifetime. For example, a MailboxProcessor may be desired to wrap some unmanaged state, such as a window or some communication session that is not thread safe, and this is not currently possible with MailboxProcessor.Start. To start a MailboxProcessor on a specific thread, simply create that thread and then call MailboxProcessor.StartImmediate on it.

Thanks goes to @xloggr for the original suggestion and PR! This PR basically just takes that PR and adds a test and the documentation for StartImmediate.

[bmitc]

I've left a self-review asking some questions that I have regarding general development, a typo in an existing test message, and primarily regarding the idea and implementation of this feature.

[0101]

Converting to draft for now. Let us know when you feel like it's ready for review.

[bmitc]

It is ready for review, which is why I converted it from draft to ready for review a week or two ago. I have been waiting for review and was planning on pinging on it today.

The feature is implemented, has a test, and I added the signatures and documentation in the mailbox.fsi file. There's nothing more to do, that I know of, besides having it reviewed.

[psfinaki]

@bmitc alright, I will take a look on this tomorrow - thanks!

[bmitc]

Sounds good! Thank you!

[psfinaki]

I am up for taking this in. I understand that the usage is limited but the API is clean, the docs and tests are fine as well.

Thanks for this!

[bmitc]

Thanks all for the help, discussion, review, and merge! 🎉