feat: add stress test projects and testing documentation

- Add String.fromChar function to convert Char to String
- Create four stress test projects demonstrating Lux features:
  - json-parser: recursive descent parsing with Char handling
  - markdown-converter: string manipulation and ADTs
  - todo-app: list operations and pattern matching
  - mini-interpreter: AST evaluation and environments
- Add comprehensive testing documentation (docs/testing.md)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types.rs

@@ -1307,6 +1307,10 @@ impl TypeEnv {
                 "substring".to_string(),
                 Type::function(vec![Type::String, Type::Int, Type::Int], Type::String),
             ),
+            (
+                "fromChar".to_string(),
+                Type::function(vec![Type::Char], Type::String),
+            ),
         ]);
         env.bind("String", TypeScheme::mono(string_module_type));