KCL: If-else expressions (#4022)

Closes https://github.com/KittyCAD/modeling-app/issues/3677

You can review each commit separately, they're neat commits with logical purpose in each.

Future enhancements:

- https://github.com/KittyCAD/modeling-app/issues/4015
- https://github.com/KittyCAD/modeling-app/issues/4020
- Right now the parser errors are not very good, especially if you forget to put an expression in the end of an if/else block

src/wasm-lib/tests/executor/inputs/no_visuals/if_else.kcl

@@ -0,0 +1,28 @@
+// This tests evaluating if-else expressions.
+
+let a = if true {
+  3
+} else if true {
+  4
+} else {
+  5
+}
+assertEqual(a, 3, 0.001, "the 'if' branch gets returned")
+
+let b = if false {
+  3
+} else if true {
+  4
+} else {
+  5
+}
+assertEqual(b, 4, 0.001, "the 'else if' branch gets returned")
+
+let c = if false {
+  3
+} else if false {
+  4
+} else {
+  5
+}
+assertEqual(c, 5, 0.001, "the 'else' branch gets returned")

src/wasm-lib/tests/executor/inputs/no_visuals/if_else_no_expr.kcl

@@ -0,0 +1,5 @@
+let x = if true {
+  let y = 1
+} else {
+  let z = 1
+}

src/wasm-lib/tests/executor/no_visuals.rs

@@ -90,4 +90,9 @@ gen_test_fail!(
     "semantic: cannot use % outside a pipe expression"
 );
 gen_test!(sketch_in_object);
+gen_test!(if_else);
+// gen_test_fail!(
+//     if_else_no_expr,
+//     "syntax: blocks inside an if/else expression must end in an expression"
+// );
 gen_test!(add_lots);