feat: typecheck If expressions

examples/sample/lib/syntax.ak

@@ -1,3 +1,7 @@
+pub type ScriptContext {
+  idk: Int
+}
+
 pub fn append(a: ByteArray, b: ByteArray) -> ByteArray {
   todo
 }

examples/sample/scripts/swap.ak

@@ -9,7 +9,13 @@ pub type Redeemer {
   Sell
 }
 
-pub fn validate(datum: Datum, rdmr: Redeemer, ctx: ScriptContext) -> Bool {
+pub fn validate(datum: Datum, rdmr: Redeemer, ctx: syntax.ScriptContext) -> Bool {
+  let thing = if True {
+    3
+  } else {
+    "thing"
+  }
+
   when rdmr is {
     Buy -> True
     Sell -> datum.something == "Aiken"