refactor: rename "process" => "proc" #30387

Problem:
- "process" is often used as a verb (`multiqueue_process_events`), which
  is ambiguous for cases where it's used as a topic.
- The documented naming convention for processes is "proc".
  - `:help dev-name-common`
- Shorter is better, when it doesn't harm readability or
  discoverability.

Solution:
Rename "process" => "proc" in all C symbols and module names.

src/nvim/api/vim.c

@@ -70,7 +70,7 @@
 #include "nvim/optionstr.h"
 #include "nvim/os/input.h"
 #include "nvim/os/os_defs.h"
-#include "nvim/os/process.h"
+#include "nvim/os/proc.h"
 #include "nvim/popupmenu.h"
 #include "nvim/pos_defs.h"
 #include "nvim/runtime.h"