add some context to the unused variable warning

jscomp/build_tests/super_errors/expected/unused_variable.res.expected

@@ -0,0 +1,14 @@
+
+  Warning number 26
+  /.../fixtures/unused_variable.res:2:7
+
+  1 │ let x = {
+  2 │   let f = 12
+  3 │   13
+  4 │ }
+
+  unused variable f.
+
+Fix this by:
+- Prepending the variable name with `_` (like `_f`) to ignore that the variable is unused.
+- Use the variable somewhere.

jscomp/build_tests/super_errors/fixtures/unused_variable.res

@@ -0,0 +1,4 @@
+let x = {
+  let f = 12
+  13
+}

jscomp/ext/warnings.ml

@@ -378,7 +378,8 @@ let message = function
   | All_clauses_guarded ->
       "this pattern-matching is not exhaustive.\n\
        All clauses in this pattern-matching are guarded."
-  | Unused_var v | Unused_var_strict v -> "unused variable " ^ v ^ "."
+  | Unused_var v | Unused_var_strict v -> 
+    Format.sprintf "unused variable %s.\n\nFix this by:\n- Prepending the variable name with `_` (like `_%s`) to ignore that the variable is unused.\n- Use the variable somewhere." v v
   | Wildcard_arg_to_constant_constr ->
       "wildcard pattern given as argument to a constant constructor"
   | Eol_in_string ->