patch 8.2.4971: Vim9: interpolated string seen as range

Problem:    Vim9: interpolated string seen as range.
Solution:   Recognize an interpolated string at the start of a command line.
            (closes #10434)

src/testdir/test_vim9_expr.vim

@@ -3572,7 +3572,9 @@ def Test_expr8_method_call()
       setline(1, ['first', 'last'])
       'second'->append(1)
       "third"->append(2)
-      assert_equal(['first', 'second', 'third', 'last'], getline(1, '$'))
+      $"fourth"->append(3)
+      $'fifth'->append(4)
+      assert_equal(['first', 'second', 'third', 'fourth', 'fifth', 'last'], getline(1, '$'))
       bwipe!
 
       var bufnr = bufnr()