Parse decimals as a unit

Fixes #257.

src/syntax/lexer.rs

@@ -430,6 +430,10 @@ impl Lexer<'_> {
         // Keep numbers and grapheme clusters together.
         if c.is_numeric() {
             self.s.eat_while(char::is_numeric);
+            let mut s = self.s;
+            if s.eat_if('.') && !s.eat_while(char::is_numeric).is_empty() {
+                self.s = s;
+            }
         } else {
             let len = self
                 .s

tests/typ/math/frac.typ

@@ -27,6 +27,7 @@ $ 1/2/3 = (1/2)/3 = 1/(2/3) $
 ---
 // Test precedence.
 $ a_1/b_2, 1/f(x), zeta(x)/2, "foo"[|x|]/2 \
+  1.2/3.7, 2.3^3.4 \
   🏳️‍🌈[x]/2, f [x]/2, phi [x]/2, 🏳️‍🌈 [x]/2 \
   +[x]/2, 1(x)/2, 2[x]/2 \
   (a)b/2, b(a)[b]/2 $