patch 8.2.1322: Vim9: method on double quoted string doesn't work

Problem:    Vim9: method on double quoted string doesn't work.
Solution:   Recognize double quoted string. (closes #6562)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index c05e98e..2546f13 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -502,8 +502,11 @@
     assert_equal('some', var)
 
     # line starting with single quote is not a mark
+    # line starting with double quote can be a method call
     'asdfasdf'->MyFunc()
     assert_equal('asdfasdf', var)
+    "xyz"->MyFunc()
+    assert_equal('xyz', var)
 
     def UseString()
       'xyork'->MyFunc()
@@ -511,6 +514,12 @@
     UseString()
     assert_equal('xyork', var)
 
+    def UseString2()
+      "knife"->MyFunc()
+    enddef
+    UseString2()
+    assert_equal('knife', var)
+
     # prepending a colon makes it a mark
     new
     setline(1, ['aaa', 'bbb', 'ccc'])