Add method to convert non-shared Collection/SourceCache to shared variants

[nicoburns]

This PR allows a Collection or SourceCache that was constructed as non-shared, to be later converted to be shared. This is useful when accepting FontContext/Collection/SourceCache provided by users (which may or may not be shared - as decided by the user) which you then want to "fan out" into thread-local shared clones.

Blitz is using the new make_shared method on SourceCache introduced in this PR. Blitz is not currently using the new corresponding make_shared method on Collection, but that method is included in this PR anyway for symmetry.

[taj-p]

I think there is a bug here. Do we need to increment the self.shared_version too or clone instead of take? Otherwise, local data seems to be lost on subsequent reads.

For example, this example code fails on the assert because all our reads are guarded through sync_shared. Since the version hasn't changed, we don't update our self.data.

use parley::fontique::{Collection, CollectionOptions};

fn main() {
    let mut collection = Collection::new(CollectionOptions {
        shared: false,
        system_fonts: false,
    });

    let font_dirs: Vec<std::path::PathBuf> = ["/System/Library/Fonts", "/Library/Fonts"]
        .iter()
        .map(std::path::PathBuf::from)
        .filter(|p| p.is_dir())
        .collect();

    if font_dirs.is_empty() {
        eprintln!("No system font directories found — cannot demonstrate the bug.");
        std::process::exit(1);
    }

    collection.load_fonts_from_paths(&font_dirs);

    let names_before: Vec<String> = collection.family_names().map(String::from).collect();

    collection.make_shared();

    let names_after: Vec<String> = collection.family_names().map(String::from).collect();

    assert_eq!(names_before.len(), names_after.len());
}