LibJS: Replace implicit environment stack with explicit registers

Replace the saved_lexical_environments stack in ExecutionContextRareData
with explicit register-based environment tracking. Environments are now
stored in registers and restored via SetLexicalEnvironment, making the
environment flow visible in bytecode.

Key changes:
- Add GetLexicalEnvironment and SetLexicalEnvironment opcodes
- CreateLexicalEnvironment takes explicit parent and dst operands
- EnterObjectEnvironment stores new environment in a dst register
- NewClass takes an explicit class_environment operand
- Remove LeaveLexicalEnvironment opcode (instead: SetLexicalEnvironment)
- Remove saved_lexical_environments from ExecutionContextRareData
- Use a reserved register for the saved lexical environment to avoid
  dominance issues with lazily-emitted GetLexicalEnvironment

Tests/LibJS/Bytecode/input/try-catch-scoping.js

@@ -0,0 +1,25 @@
+function tryCatchWithBlocks() {
+    let x = 1;
+    try {
+        let y = 2;
+        throw y;
+    } catch (e) {
+        let z = 3;
+        console.log(x + e + z);
+    }
+}
+tryCatchWithBlocks();
+
+function tryCatchFinallyWithBlocks() {
+    let x = 1;
+    try {
+        let y = 2;
+        throw y;
+    } catch (e) {
+        console.log(e);
+    } finally {
+        let z = 3;
+        console.log(z);
+    }
+}
+tryCatchFinallyWithBlocks();