Public dependency on `once_cell`

[CosmicHorrorDev]

Now that there are things like OnceLock (since 1.70.0) and LazyLock(since 1.80.0) in the standard library it'd be nice to migrate off of once_cell. The one current issue with this is that it's a breaking change for this library due to syntect::parsing::SCOPE_REPO being a part of the public API while it's a once_cell::sync::Lazy. It'd be nice to change this in the next breaking release if possible (and I'd be willing to open a PR to handle it)

[keith-hall]

I wonder if anyone ever actually uses syntect::parsing::SCOPE_REPO, most likely I expect it wouldn't be a big deal to change it and release a new major version as it would be trivial to upgrade 🤞

[Enselic]

Also see #512.

I'm not against releasing a new major version, but then I think we should try to fix the issues listed in https://github.com/trishume/syntect/milestone/1 and maybe go through new issues since then and see if there is more to do. It would be nice to not have to release 7.0 shortly after 6.0.

[CosmicHorrorDev]

Well since it sounds like everyone is on board with getting rid of SCOPE_REPO in it's current iteration I'll go ahead and open a PR that can officially mark it #[deprecated] which can get out in the next non-breaking release. I'm fine with #576 hanging around till whenever things are ready for 6.0

I can also spend some of my freetime over the next couple of months working towards the 6.0 milestone. #512 does look like a good place to start