Introduce the tab_width setting and give tabs a greater value when counting indentation level.

src/types/greater_element.rs

@@ -10,10 +10,13 @@ pub struct PlainList<'s> {
     pub children: Vec<PlainListItem<'s>>,
 }
 
+/// The width that something is indented. For example, a single tab character could be a value of 4 or 8.
+pub type IndentationLevel = u16;
+
 #[derive(Debug)]
 pub struct PlainListItem<'s> {
     pub source: &'s str,
-    pub indentation: usize,
+    pub indentation: IndentationLevel,
     pub bullet: &'s str,
     pub checkbox: Option<(CheckboxType, &'s str)>,
     pub tag: Vec<Object<'s>>,

src/types/mod.rs

@@ -16,6 +16,7 @@ pub use greater_element::Drawer;
 pub use greater_element::DynamicBlock;
 pub use greater_element::FootnoteDefinition;
 pub use greater_element::GreaterBlock;
+pub use greater_element::IndentationLevel;
 pub use greater_element::NodeProperty;
 pub use greater_element::PlainList;
 pub use greater_element::PlainListItem;