Add chrono dependency and example files

This commit introduces the `chrono` dependency to the project, enabling
the use of time-related functionalities. Additionally, it adds several
example files (`closure.myc`, `fib.myc`, `hof.myc`) to showcase the
language's features like closures, recursion, and higher-order
functions.

The `Cargo.lock` and `Cargo.toml` files have been updated to reflect the
new dependency. The `integration_test.rs` file now includes a test to
run all `.myc` files found in the `examples` directory, ensuring their
correctness. The `main.rs` file has also been updated to load and
display these examples in the UI.

examples/hof.myc

@@ -0,0 +1,8 @@
+;; Higher-Order Function Example
+;; Output: 25
+(do
+  (def apply (fn [f x] (f x)))
+  (def square (fn [x] (* x x)))
+
+  (apply square 5)
+)