feat(emacs/lean-input): add shortcuts for subscripts

src/emacs/lean-input.el

@@ -221,6 +221,17 @@ order for the change to take effect."
   ("m="   . ("≞"))
   ("?="   . ("≟"))
 
+  ("1" . ("₁"))
+  ("2" . ("₂"))
+  ("3" . ("₃"))
+  ("4" . ("₄"))
+  ("5" . ("₅"))
+  ("6" . ("₆"))
+  ("7" . ("₇"))
+  ("8" . ("₈"))
+  ("9" . ("₉"))
+  ("0" . ("₀"))
+
   ;; Inequality and similar symbols.
 
   ("leq"  . ,(lean-input-to-string-list "<≪⋘≤≦≲ ≶≺≼≾⊂⊆ ⋐⊏⊑ ⊰⊲⊴⋖⋚⋜⋞"))