Extract shell_skip_word from mch_call_shell

src/os/shell.c

@@ -0,0 +1,23 @@
+#include <stdbool.h>
+
+#include "os/shell.h"
+#include "types.h"
+#include "ascii.h"
+
+
+void shell_skip_word(char_u **ptr)
+{
+  char_u *p = *ptr;
+  bool inquote = false;
+
+  // Move `p` to the end of shell word by advancing the pointer it while it's
+  // inside a quote or it's a non-whitespace character
+  while (*p && (inquote || (*p != ' ' && *p != TAB))) {
+    if (*p == '"')
+      // Found a quote character, switch the `inquote` flag
+      inquote = !inquote;
+    ++p;
+  }
+
+  *ptr = p;
+}