feat: implement --new flag

The new `--new` or `-n` flag allows to create a new revision after the
single revision being described. Running `jj-cz --new` is the
equivalent of running `jj-cz @ && jj new`. Running `jj-cz --new xs` is
the equivalent of running `jj-cz xs && jj new xs`.

Passing several revisions to `jj-cz` with the `--new` flag will result
in an error.

Refs: #6

src/error.rs

@@ -31,6 +31,8 @@ pub enum Error {
     RevsetResolutionError { revset: String, context: String },
     #[error("Revision set '{revset}' resolves to multiple commits; specify a single revision")]
     MultipleRevisions { revset: String },
+    #[error("--new cannot be used with multiple revisions")]
+    NewFlagWithMultipleRevisions,
 }
 
 impl From<ScopeError> for Error {