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/strings/size.rs

@@ -359,7 +359,7 @@ fn test_count_counts_lines() {
     const LS: &str = "\u{2028}"; // 0xe280a8
     const PS: &str = "\u{2029}"; // 0xe280a9
 
-    // * \r\n is a single graheme cluster
+    // * \r\n is a single grapheme cluster
     // * trailing newlines are counted
     // * NEL is 2 bytes
     // * FF is 1 byte