runtime(vim): Update base-syntax, improve :echo highlighting (#14103)

- Normalise behaviour of :echo commands and improve expression matching.
- Allow continued argument lines.
- Refine string interpolation groups.
- Remove duplicated :menu and :map generated commands that are handled
  specially later in the file.

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim_ex_echo.vim

@@ -0,0 +1,20 @@
+" Vim :echo commands
+
+echo        "Answer = " 42
+echon       "Answer = " 42
+echomsg     "Answer = " 42
+echowindow  "Answer = " 42
+echoerr     "Answer = " 42
+echoconsole "Answer = " 42
+
+echo "following command is :|" | |
+
+echohl WarningMsg | echo "Don't panic!" | echohl None
+
+echo "Answer = "
+     "\ comment
+      \ 42
+
+" :echo without {expr}
+echo| echo "Foo"
+