rename unfold to generate (#10770)

# Description

This PR renames the `unfold` command to `generate`.
closes #10760

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

@@ -33,6 +33,7 @@ mod flatten;
 mod for_;
 #[cfg(feature = "extra")]
 mod format;
+mod generate;
 mod get;
 mod glob;
 mod group_by;
@@ -103,7 +104,6 @@ mod touch;
 mod transpose;
 mod try_;
 mod ucp;
-mod unfold;
 mod uniq;
 mod uniq_by;
 mod update;