Fix typos and use more idiomatic assertions (#7755)

I have changed `assert!(a == b)` calls to `assert_eq!(a, b)`, which give
better error messages. Similarly for `assert!(a != b)` and
`assert_ne!(a, b)`. Basically all instances were comparing primitives
(string slices or integers), so there is no loss of generality from
special-case macros,

I have also fixed a number of typos in comments, variable names, and a
few user-facing messages.

crates/nu-command/src/dataframe/expressions/otherwise.rs

@@ -23,7 +23,7 @@ impl Command for ExprOtherwise {
             .required(
                 "otherwise expression",
                 SyntaxShape::Any,
-                "expressioini to apply when no when predicate matches",
+                "expression to apply when no when predicate matches",
             )
             .input_type(Type::Any)
             .output_type(Type::Custom("expression".into()))