[WASM] Initial infrastructure

- New module ":compiler:backend.wasm"
    - Initial compiler infra (driver, phaser, context)
    - Subset of Wasm AST
    - Skeleton of IR -> Wasm AST
    - Wasm AST -> WAT transformer

- Testing infra

- SpiderMonkey jsshell tool

js/js.translator/testData/wasmBox/passedCommonTests/when/noElseNoMatch.kt

@@ -0,0 +1,8 @@
+fun box(): String {
+    val x = 3
+    when (x) {
+        1 -> {}
+        2 -> {}
+    }
+    return "OK"
+}