patch 8.2.1490: Vim9: using /= with float and number doesn't work

Problem:    Vim9: using /= with float and number doesn't work.
Solution:   Better support assignment with operator. (closes #6742)

src/testdir/test_vim9_script.vim

@@ -90,6 +90,18 @@ def Test_assignment()
   &ts %= 4
   assert_equal(2, &ts)
 
+  if has('float')
+    let f100: float = 100.0
+    f100 /= 5
+    assert_equal(20.0, f100)
+
+    let f200: float = 200.0
+    f200 /= 5.0
+    assert_equal(40.0, f200)
+
+    CheckDefFailure(['let nr: number = 200', 'nr /= 5.0'], 'E1012:')
+  endif
+
   lines =<< trim END
     vim9script
     &ts = 6