Make if blocks work like a def-env (#4656)

* Make `if` work like a def-env

* Add some tests

* Add an example

src/tests/test_engine.rs

@@ -350,3 +350,16 @@ fn default_value11() -> TestResult {
 fn default_value12() -> TestResult {
     fail_test(r#"def foo [--x:int = "a"] { $x }"#, "default value not int")
 }
+
+#[test]
+fn def_env_if() -> TestResult {
+    run_test(r#"if true { let-env FOO = "QUX" }; $env.FOO"#, "QUX")
+}
+
+#[test]
+fn def_env_if2() -> TestResult {
+    run_test(
+        r#"if false { let-env FOO = "QUX" } else { let-env FOO = "QUUX" }; $env.FOO"#,
+        "QUUX",
+    )
+}