feat: Add a way to keep lines with no annotations

[Muscraft]

This PR makes it so that you can "annotate" text with AnnotationKind::Visible to keep it from getting folded, allowing you to keep some lines around as context without underlining them with annotation characters.

error: functions are not allowed in struct definitions
  --> $DIR/struct-fn-in-definition.rs:9:5
   |
LL | struct S {
...
LL |     fn foo() {}
   |     ^^^^^^^^^^^
   |
   = help: unlike in C++, Java, and C#, functions are declared in `impl` blocks