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-command/src/default_context.rs

@@ -197,7 +197,8 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             StrSubstring,
             StrTrim,
             StrTitleCase,
-            StrUpcase
+            StrUpcase,
+            FormatDate
         };
 
         // FileSystem
@@ -233,7 +234,6 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
         // Date
         bind_command! {
             Date,
-            DateFormat,
             DateHumanize,
             DateListTimezones,
             DateNow,
@@ -300,6 +300,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
         // Env
         bind_command! {
             LetEnvDeprecated,
+            DateFormat,
             ExportEnv,
             LoadEnv,
             SourceEnv,