Opens an editor for constructing a user prompt, using the currently defined chat mode.
The editor is determined as follows:
Look for the following environment variables, in order:
1. AIDER_EDITOR
2. VISUAL
3. EDITOR
If none of these are defined, use the following defaults:
Windows: notepad
OS X: vim
*nix: vi
If an editor is not found, a RuntimeError is raised.
Any arguments passed after the /editor command are inserted as content.
The temporary file used for editing has an .md extension, which can be leveraged for syntax highlighting.
NOTE: The editor used MUST block the process until the editor is closed -- the default editors all do this.
