aider: Added /exit command to aider/commands.py.

# Context:
USER: add an /exit command which calls sys.exit
ASSISTANT: To add an `/exit` command, we need to modify the `commands.py` file to add a new command method and update the `is_command` and `run` methods to recognize and execute the new command. We also need to import `sys` to use `sys.exit()`.

Here are the changes:

```python
aider/commands.py
<<<<<<< ORIGINAL
from . import io
=======
import sys
from . import io
>>>>>>> UPDATED
```

```python
aider/commands
^C KeyboardInterrupt
USER: I added the content of these additional files: aider/commands.py
ASSISTANT: I apologize for the confusion. Here are the changes to add the `/exit` command to the `aider/commands.py` file:

```python
aider/commands.py
<<<<<<< ORIGINAL
import os
import git
import subprocess
import shlex
=======
import sys
import os
import git
import subprocess
import shlex
>>>>>>> UPDATED
```

```python
aider/commands.py
<<<<<<< ORIGINAL
        commands = ["/help"]
=======
        commands = ["/help", "/exit"]
>>>>>>> UPDATED
```

```python
aider/commands.py
<<<<<<< ORIGINAL
        else:
            self.io.tool_error(f"Error: {first_word} is not a valid command.")
=======
        elif first_word == "/exit":
            self.cmd_exit()
        else:
            self.io.tool_error(f"Error: {first_word} is not a valid command.")
>>>>>>> UPDATED
```

```python
aider/commands.py
<<<<<<< ORIGINAL
    def cmd_ls(self, args):
        "List all known files and those included in the chat session"
=======
    def cmd_exit(self):
        "Exit the application"
        sys.exit()

    def cmd_ls(self, args):
        "List all known files and those included in the chat session"
>>>>>>> UPDATED
```

These changes will add the `/exit` command to the list of commands, recognize it when entered, and call the `cmd_exit` method to exit the application using `sys.exit()`.

aider/commands.py

@@ -1,3 +1,4 @@
+import sys
 import os
 import git
 import subprocess
@@ -29,7 +30,7 @@ class Commands:
                 self.io.tool(f"{cmd} No description available.")
 
     def get_commands(self):
-        commands = ["/help"]
+        commands = ["/help", "/exit"]
         for attr in dir(self):
             if attr.startswith("cmd_"):
                 commands.append("/" + attr[4:])
@@ -68,6 +69,8 @@ class Commands:
                 return self.do_run(matching_commands[0][1:], rest_inp)
         elif len(matching_commands) > 1:
             self.io.tool_error("Ambiguous command: ', '.join(matching_commands)}")
+        elif first_word == "/exit":
+            self.cmd_exit()
         else:
             self.io.tool_error(f"Error: {first_word} is not a valid command.")
 
@@ -251,6 +254,10 @@ class Commands:
         )
         return msg
 
+    def cmd_exit(self):
+        "Exit the application"
+        sys.exit()
+
     def cmd_ls(self, args):
         "List all known files and those included in the chat session"