Quote strings containing = (#16440)

Fixes nushell/nushell#16438

## Release notes summary - What our users need to know

Previously, a record key `=` would not be quoted in `to nuon`, producing
invalid nuon output. It wasn't quoted in other string values either, but
it didn't cause problems there. Now any string containing `=` gets
quoted.

crates/nu-command/tests/format_conversions/nuon.rs

@@ -515,6 +515,19 @@ fn quotes_some_strings_necessarily() {
     assert_eq!(actual.out, "list<string>");
 }
 
+#[test]
+fn quotes_some_strings_necessarily_in_record_keys() {
+    let actual = nu!(pipeline(
+        r#"
+        ['=', 'a=', '=a'] | each {
+           {$in : 42}
+        } | reduce {|elt, acc| $acc | merge $elt} | to nuon | from nuon | columns | describe
+        "#
+    ));
+
+    assert_eq!(actual.out, "list<string>");
+}
+
 #[test]
 fn read_code_should_fail_rather_than_panic() {
     let actual = nu!(cwd: "tests/fixtures/formats", pipeline(

crates/nu-utils/src/quoting.rs

@@ -2,12 +2,12 @@ use fancy_regex::Regex;
 use std::sync::LazyLock;
 
 // This hits, in order:
-// • Any character of []:`{}#'";()|$,.!?
+// • Any character of []:`{}#'";()|$,.!?=
 // • Any digit (\d)
 // • Any whitespace (\s)
 // • Case-insensitive sign-insensitive float "keywords" inf, infinity and nan.
 static NEEDS_QUOTING_REGEX: LazyLock<Regex> = LazyLock::new(|| {
-    Regex::new(r#"[\[\]:`\{\}#'";\(\)\|\$,\.\d\s!?]|(?i)^[+\-]?(inf(inity)?|nan)$"#)
+    Regex::new(r#"[\[\]:`\{\}#'";\(\)\|\$,\.\d\s!?=]|(?i)^[+\-]?(inf(inity)?|nan)$"#)
         .expect("internal error: NEEDS_QUOTING_REGEX didn't compile")
 });