Fix building with 1.72

[emilk]

The reported MSRV of this project is 1.72, but rustc 1.72 fails to compile it:

error[E0446]: private type `CommonMarkOptions` in public interface
   --> src/parsers/pulldown.rs:57:5
    |
57  | /     pub fn show(
58  | |         &mut self,
59  | |         ui: &mut egui::Ui,
60  | |         cache: &mut CommonMarkCache,
...   |
63  | |         populate_split_points: bool,
64  | |     ) {
    | |_____^ can't leak private type
    |
   ::: src/lib.rs:240:1
    |
240 |   struct CommonMarkOptions {
    |   ------------------------ `CommonMarkOptions` declared as private

This PR fixes the compilation error, and enforces testing with 1.72 using a rust-toolchain file, which the CI will heed, as will any local cargo check.

This should probably have a patch release.

[lampsitter]

The struct is not supposed to be public. Instead the function in the pulldown parser should be pub(crate). The newer compiler must have gotten smart enough to understand that the functions are within a private module. I'll make an alternative fix

[lampsitter]

Patch has been released