Improve error messages

Using "codepoint" is more accurate and lines up with what typst's standard library uses

crates/typst-library/src/math/accent.rs

@@ -189,6 +189,6 @@ cast! {
     v: char => Self::new(v),
     v: Content => match v.to_packed::<SymbolElem>() {
         Some(elem) if elem.text.chars().count() == 1 => Self::new(elem.text.chars().next().unwrap()),
-        _ => bail!("expected a single-character symbol"),
+        _ => bail!("expected a single-codepoint symbol"),
     },
 }

crates/typst-library/src/math/matrix.rs

@@ -274,7 +274,7 @@ cast! {
     Delimiter,
     self => self.0.into_value(),
     _: NoneValue => Self::none(),
-    v: Symbol => Self::char(v.get().parse::<char>().map_err(|_| "symbol value is longer than one character")?)?,
+    v: Symbol => Self::char(v.get().parse::<char>().map_err(|_| "expected a single-codepoint symbol")?)?,
     v: char => Self::char(v)?,
 }