feat(es/compiler): Merge optional_chaining into Compiler

This PR merges the `optional_chaining` visitor from `swc_ecma_compat_es2020` into `swc_ecma_compiler::Compiler` to reduce visitor overhead and improve performance by consolidating ECMAScript compatibility transformations.

## Changes

### 1. Created ES2020 Module
- Created `crates/swc_ecma_compiler/src/es2020/optional_chaining.rs` with transformation logic
- Updated `crates/swc_ecma_compiler/src/es2020/mod.rs` to include the new module

### 2. Integrated into CompilerImpl
Added transformation state to `CompilerImpl`:
- `es2020_optional_chaining_vars: Vec<VarDeclarator>` - stores generated variable declarators
- `es2020_optional_chaining_unresolved_ctxt: SyntaxContext` - tracks unresolved context

Implemented visitor methods:
- `transform_optional_chaining()` - transforms `?.` and `delete ?.` operators
- `visit_mut_expr()` - calls transformation before other transformations
- `visit_mut_block_stmt_or_expr()` - converts expressions to block statements when needed
- `visit_mut_pat()` - handles optional chaining in assignment patterns
- `visit_mut_module_items()` / `visit_mut_stmts()` - hoist generated variables

### 3. Updated swc_ecma_compat_es2020
- Modified `lib.rs` to use `Compiler` with `Features::OPTIONAL_CHAINING`
- Simplified `optional_chaining.rs` to a thin wrapper around `Compiler`
- Maintained backward compatibility and existing `Config` API
- Maps `pure_getter` config to `assumptions.pure_getters`

## Testing

All existing tests pass:
- ✅ `cargo test -p swc_ecma_compat_es2020`
- ✅ `cargo test -p swc_ecma_compiler`

## Performance Impact

This change is part of a larger effort to reduce visitor overhead by consolidating transformations into a single Compiler implementation, minimizing the number of AST traversals.

## Related PRs

Follows the same pattern as:
- #11157: Merged nullish_coalescing
- #10914: Merged logical_assignments
- #10909: Merged private_in_object and static_blocks

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: github-actions[bot]

crates/swc_ecma_compat_es2020/src/lib.rs

@@ -13,18 +13,18 @@ mod export_namespace_from;
 pub mod nullish_coalescing;
 pub mod optional_chaining;
 
-pub fn es2020(config: Config, unresolved_mark: Mark) -> impl Pass {
+pub fn es2020(config: Config, _unresolved_mark: Mark) -> impl Pass {
     let mut assumptions = Assumptions::default();
     assumptions.no_document_all = config.nullish_coalescing.no_document_all;
+    assumptions.pure_getters = config.optional_chaining.pure_getter;
 
-    (
-        optional_chaining(config.optional_chaining, unresolved_mark),
-        Compiler::new(swc_ecma_compiler::Config {
-            includes: Features::EXPORT_NAMESPACE_FROM | Features::NULLISH_COALESCING,
-            assumptions,
-            ..Default::default()
-        }),
-    )
+    Compiler::new(swc_ecma_compiler::Config {
+        includes: Features::EXPORT_NAMESPACE_FROM
+            | Features::NULLISH_COALESCING
+            | Features::OPTIONAL_CHAINING,
+        assumptions,
+        ..Default::default()
+    })
 }
 
 #[derive(Debug, Clone, Default, Deserialize)]

crates/swc_ecma_compat_es2020/src/optional_chaining.rs

@@ -1,17 +1,19 @@
 use serde::Deserialize;
 use swc_common::Mark;
 use swc_ecma_ast::Pass;
-use swc_ecma_compat_es2022::optional_chaining_impl::optional_chaining_impl;
-use swc_ecma_visit::visit_mut_pass;
+use swc_ecma_compiler::{Compiler, Features};
+use swc_ecma_transforms_base::assumptions::Assumptions;
 
-pub fn optional_chaining(c: Config, unresolved_mark: Mark) -> impl Pass {
-    visit_mut_pass(optional_chaining_impl(
-        swc_ecma_compat_es2022::optional_chaining_impl::Config {
-            no_document_all: c.no_document_all,
-            pure_getter: c.pure_getter,
-        },
-        unresolved_mark,
-    ))
+pub fn optional_chaining(c: Config, _unresolved_mark: Mark) -> impl Pass {
+    let mut assumptions = Assumptions::default();
+    assumptions.no_document_all = c.no_document_all;
+    assumptions.pure_getters = c.pure_getter;
+
+    Compiler::new(swc_ecma_compiler::Config {
+        includes: Features::OPTIONAL_CHAINING,
+        assumptions,
+        ..Default::default()
+    })
 }
 
 #[derive(Debug, Clone, Copy, Default, Deserialize)]

crates/swc_ecma_compiler/src/es2020/mod.rs

@@ -1,2 +1,3 @@
 pub(crate) mod export_namespace_from;
 pub(crate) mod nullish_coalescing;
+pub(crate) mod optional_chaining;