[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

compiler/tests-common/tests/org/jetbrains/kotlin/test/TargetBackend.kt

@@ -12,7 +12,8 @@ enum class TargetBackend(
     JVM,
     JVM_IR(JVM),
     JS,
-    JS_IR(JS);
+    JS_IR(JS),
+    WASM;
 
     val compatibleWith get() = compatibleWithTargetBackend ?: ANY
 }