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 🤞