Rename cond math (#2807)

* Simplifies 'if' to work on the available scope rather than a stream

* Rename initializer/math for better readability

* Fix description

* fmt

crates/nu-cli/src/commands/keep/until.rs

@@ -18,7 +18,7 @@ impl WholeStreamCommand for SubCommand {
         Signature::build("keep until")
             .required(
                 "condition",
-                SyntaxShape::Math,
+                SyntaxShape::RowCondition,
                 "The condition that must be met to stop keeping rows",
             )
             .filter()