rename from date format to format date (#9902)

# Description
Closes: #9891
I also think it's good to keep command name consistency.

And moving `date format` to deprecated.

# User-Facing Changes
Running `date format` will lead to deprecate message:
```nushell
❯ "2021-10-22 20:00:12 +01:00" | date format
Error: nu:🐚:deprecated_command

  × Deprecated command date format
   ╭─[entry #28:1:1]
 1 │ "2021-10-22 20:00:12 +01:00" | date format
   ·                                ─────┬─────
   ·                                     ╰── 'date format' is deprecated. Please use 'format date' instead.
   ╰────
```

crates/nu-std/std/log.nu

@@ -139,7 +139,7 @@ def current-log-level [] {
 }
 
 def now [] {
-    date now | date format "%Y-%m-%dT%H:%M:%S%.3f"
+    date now | format date "%Y-%m-%dT%H:%M:%S%.3f"
 }
 
 def handle-log [