Add Scala 3 build

[xuwei-k]

No description provided.

[sjrd]

For Scala 3 support, I would prefer a single PR with multiple commits: one for each kind of required change in the code, and finally one that adds building with Scala 3. Otherwise we can't really test whether the changes actually work.

[xuwei-k]

• this.synchronized block prevents @tailrec check from passing scala/scala3#14308

[xuwei-k]

[error] 122 |final object RunConfig {
[error]     |^^^^^
[error]     |Modifier final is redundant for this definition

[sjrd]

The code changes look good. I only have nits on the commit messages:

• Please directly include your self-review comments in the relevant commit messages. They justify the existence of those commits, so they are important.
• While you're at it, ideally follow the commit message style we use, which starts with an uppercase (e.g., "Add Scala 3 build" instead of "add").