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/os/pty_proc_unix.h

@@ -0,0 +1,18 @@
+#pragma once
+// IWYU pragma: private, include "nvim/os/pty_proc.h"
+
+#include <stdint.h>
+#include <sys/ioctl.h>
+
+#include "nvim/event/defs.h"
+
+typedef struct {
+  Proc proc;
+  uint16_t width, height;
+  struct winsize winsize;
+  int tty_fd;
+} PtyProc;
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/pty_proc_unix.h.generated.h"
+#endif