Fix lint issues and improve test coverage for Unmarshaler interface

- Apply De Morgan's law in keyNeedsQuoting to satisfy staticcheck QF1001
- Remove unused splitTableUnmarshaler type from test
- Fix unused parameter lint warning in errorUnmarshaler873
- Add test for quoted keys that need special handling
- Add test for error propagation from UnmarshalTOML
- Update customTable873 parser to handle quoted keys properly

Coverage improved:
- handleKeyValuesUnmarshaler: 80.0% -> 93.3%
- keyNeedsQuoting: 66.7% -> 83.3%
- Overall main package: 97.2% -> 97.5%

unmarshaler.go

@@ -749,8 +749,8 @@ func keyNeedsQuoting(key []byte) bool {
 	}
 	for _, b := range key {
 		// Bare keys can only contain A-Za-z0-9_-
-		if !((b >= 'A' && b <= 'Z') || (b >= 'a' && b <= 'z') ||
-			(b >= '0' && b <= '9') || b == '_' || b == '-') {
+		if (b < 'A' || b > 'Z') && (b < 'a' || b > 'z') &&
+			(b < '0' || b > '9') && b != '_' && b != '-' {
 			return true
 		}
 	}