feat: Add SWC feature flags library with build-time and runtime transforms (#578)

Implements a two-phase feature flag system:

Phase 1 (Build-time):
- SWC Wasm plugin that marks feature flag usage
- Replaces flag identifiers with __SWC_FLAGS__ markers
- Removes imports and hook calls from configured libraries
- Supports destructuring patterns and multiple libraries
- Handles variable shadowing correctly using SWC's Id system

Phase 2 (Runtime):
- Standalone Rust crate for dead code elimination
- Substitutes flag values with boolean literals
- Eliminates dead branches (if, ternary, logical operators)
- Tracks statistics (bytes removed, branches eliminated)
- Unwraps block statements for cleaner output

Features:
- Customizable function names (not hardcoded)
- Selective flag exclusion
- Scope-safe transformations
- Minifier-safe markers
- Comprehensive fixture tests
- TypeScript definitions

Packages:
- crates/swc_feature_flags: Standalone Rust crate
- packages/feature-flags: SWC Wasm plugin wrapper

---------

Co-authored-by: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>
Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Donny/강동윤 <kdy1@users.noreply.github.com>

Author: 3 people

Cargo.lock

@@ -0,0 +1,26 @@
+[package]
+description = "Feature flag transformation for SWC - build-time marking and runtime dead code elimination"
+name        = "swc_feature_flags"
+version     = "0.1.0"
+
+authors      = { workspace = true }
+edition      = { workspace = true }
+homepage     = { workspace = true }
+license      = { workspace = true }
+repository   = { workspace = true }
+rust-version = { workspace = true }
+
+[dependencies]
+serde            = { workspace = true, features = ["derive"] }
+swc_atoms        = { workspace = true }
+swc_common       = { workspace = true }
+swc_ecma_ast     = { workspace = true }
+swc_ecma_codegen = { workspace = true }
+swc_ecma_visit   = { workspace = true }
+
+[dev-dependencies]
+swc_ecma_codegen            = { workspace = true }
+swc_ecma_parser             = { workspace = true }
+swc_ecma_transforms_base    = { workspace = true }
+swc_ecma_transforms_testing = { workspace = true }
+testing                     = { workspace = true }