Add an initial list, for testing purposes

[DilumAluthge]

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[DilumAluthge]

This is the repo that would house the list mentioned in JuliaLang/julia#58303 (comment) ("a separately maintained opt-in list") and JuliaLang/julia#58303 (comment) ("We could also have a file somewhere that is a list of committers to choose from when assigning people to PRs").

I figured that if we host this in a separate repo, then we can reduce traffic on the main JuliaLang/julia repo. However, if we want to keep this file in the main JuliaLang/julia repo instead, then I think that's also fine.

If we were able to use the CODEOWNERS approach (JuliaLang/julia#58315, reverted in JuliaLang/julia#58358), then I think we could have kept this list in a GitHub team instead, because CODEOWNERS supports GitHub teams. However, we aren't able to use CODEOWNERS (because it lacks the features discussed in JuliaLang/julia#58358 (comment)). So, because we are instead using GitHub Actions, I think a simple text file works fine.

(Because we are using GitHub Actions, pulling info from a GitHub team via the GitHub API is quite complicated and has various security challenges - PATs are pretty strongly discouraged by GitHub these days, so we'd have to set up a custom GitHub App, and then use the GitHub App's secrets to generate tokens, which is a big pain for relatively little payoff.)

[DilumAluthge]

@LilithHafner I'd suggest we do the following:

1. Merge this PR.
2. Use the users.txt file in this repo to maintain the list of commiters that are opting-in. I've already given all Julia committers write access to this repo, so anyone can easily add or remove themselves.
3. In CI: Automatically assign a committer to PRs not opened by committers julia#58303, in your GitHub Action, you can use the GitHub API to pull the contents of the users.txt from the latest commit on the main branch. You basically want this endpoint: https://docs.github.com/en/rest/repos/contents?apiVersion=2022-11-28#get-repository-content
4. Once you get the file contents, split by newline (and remove the @ signs).