refactor(test): deprecate n.feed_command() #32915

Problem:
`feed_command()` was added as a "bridge" for old test code. 99% of those
cases should be using `n.command()`, which raises errors instead of
silently continuing the test.

Solution:
Deprecate `feed_command()`. It should not be used in new tests.
All usages of `feed_command()` should be converted to `command()` or
`feed()`.

runtime/doc/news.txt

@@ -70,12 +70,6 @@ BUILD
 • Translations are turned off by default. Enable by building Nvim with the
   CMake flag `ENABLE_TRANSLATIONS=ON`.
 
-DEFAULTS
-
-• 'number', 'relativenumber', 'signcolumn', and 'foldcolumn' are disabled in
-  |terminal| buffers. See |terminal-config| for an example of changing these defaults.
-• 'diffopt' default includes "linematch:40".
-
 DIAGNOSTICS
 
 • The "underline" diagnostics handler sorts diagnostics by severity when using
@@ -247,6 +241,11 @@ DEFAULTS
   • |[[| and |]]| in Normal mode jump between shell prompts for shells which emit
     OSC 133 sequences ("shell integration" or "semantic prompts").
 
+• Options:
+  • 'diffopt' default includes "linematch:40".
+  • 'number', 'relativenumber', 'signcolumn', and 'foldcolumn' are disabled in
+    |terminal| buffers. |terminal-config| shows how to change these defaults.
+
 • Snippet:
   • `<Tab>` in Insert and Select mode maps to `vim.snippet.jump({ direction = 1 })`
     when a snippet is active and jumpable forwards.