patch 8.1.0553: it is not easy to edit a script that was sourced

Problem:    It is not easy to edit a script that was sourced.
Solution:   Add a count to ":scriptnames", so that ":script 40" edits the
            script with script ID 40.

src/testdir/test_scriptnames.vim

@@ -0,0 +1,26 @@
+" Test for :scriptnames
+
+func Test_scriptnames()
+  call writefile(['let did_load_script = 123'], 'Xscripting')
+  source Xscripting
+  call assert_equal(123, g:did_load_script)
+
+  let scripts = split(execute('scriptnames'), "\n")
+  let last = scripts[-1]
+  call assert_match('\<Xscripting\>', last)
+  let lastnr = substitute(last, '\D*\(\d\+\):.*', '\1', '')
+  exe 'script ' . lastnr
+  call assert_equal('Xscripting', expand('%:t'))
+
+  call assert_fails('script ' . (lastnr + 1), 'E474:')
+  call assert_fails('script 0', 'E939:')
+
+  new
+  call setline(1, 'nothing')
+  call assert_fails('script ' . lastnr, 'E37:')
+  exe 'script! ' . lastnr
+  call assert_equal('Xscripting', expand('%:t'))
+
+  bwipe
+  call delete('Xscripting')
+endfunc