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()`.

test/functional/testnvim.lua

@@ -620,6 +620,8 @@ function M.insert(...)
   nvim_feed('<ESC>')
 end
 
+--- @deprecated Use `command()` or `feed()` instead.
+---
 --- Executes an ex-command by user input. Because nvim_input() is used, Vimscript
 --- errors will not manifest as client (lua) errors. Use command() for that.
 --- @param ... string