docs: documnt command.preexec hook

docs/hooks/command.md

@@ -1,3 +1,8 @@
++ `command.preexec` -> input, cmdStr > Thrown before a command
+is executed. The `input` is the user written command, while `cmdStr`
+is what will be executed (`input` will have aliases while `cmdStr`
+will have alias resolved input).
+
 + `command.exit` -> code, cmdStr > Thrown when a command exits.
 `code` is the exit code of the command, and `cmdStr` is the command that was run.