add table -> table to into datetime (#9775)

should close https://github.com/nushell/nushell/issues/9774

# Description
given the help page of `into datetime`, 
```
Parameters:
  ...rest <cellpath>: for a data structure input, convert data at the given cell paths
```
it looks like `into datetime` should accept tables as input 🤔 

this PR
- adds the `table -> table` signature to `into datetime`
- adds a test to make sure the behaviour stays there

crates/nu-command/tests/commands/mod.rs

@@ -39,6 +39,7 @@ mod help;
 mod histogram;
 mod insert;
 mod inspect;
+mod into_datetime;
 mod into_filesize;
 mod into_int;
 mod join;