[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/controlStructures/ifConst1.kt

@@ -0,0 +1,19 @@
+fun foo(b: Boolean): String {
+    return if (b) {
+        "OK"
+    } else if (false) {
+        "fail: reached unreachable code at line 5"
+    } else if (true) {
+        "fail: reached unexpected code at line 7"
+    } else if (true) {
+        "fail: reached unreachable code at line 9"
+    } else if (b) {
+        "fail: reached unreachable code at line 11"
+    } else {
+        "fail: reached unreachable code at line 13"
+    }
+}
+
+fun box(): String {
+    return foo(true)
+}

js/js.translator/testData/wasmBox/passedCommonTests/controlStructures/ifConst2.kt

@@ -0,0 +1,19 @@
+fun foo(b: Boolean): String {
+    return if (b) {
+        "fail: reached unexpected code at line 3"
+    } else if (false) {
+        "fail: reached unreachable code at line 5"
+    } else if (true) {
+        "OK"
+    } else if (true) {
+        "fail: reached unreachable code at line 9"
+    } else if (b) {
+        "fail: reached unreachable code at line 11"
+    } else {
+        "fail: reached unreachable code at line 13"
+    }
+}
+
+fun box(): String {
+    return foo(false)
+}

js/js.translator/testData/wasmBox/passedCommonTests/controlStructures/kt1899.kt

@@ -0,0 +1,5 @@
+fun box(): String {
+  if (1 != 0) {
+  }
+  return "OK"
+}