Implement both enum-based and nan-boxed JsValue inner behind a flag

cli/Cargo.toml

@@ -28,6 +28,7 @@ dhat = { workspace = true, optional = true }
 [features]
 default = ["boa_engine/annex-b", "boa_engine/experimental", "boa_engine/intl_bundled"]
 dhat = ["dep:dhat"]
+nan-box-jsvalue = ["boa_engine/nan-box-jsvalue"]
 
 [target.x86_64-unknown-linux-gnu.dependencies]
 jemallocator.workspace = true

core/engine/Cargo.toml

@@ -15,6 +15,7 @@ rust-version.workspace = true
 profiler = ["boa_profiler/profiler"]
 deser = ["boa_interner/serde", "boa_ast/serde"]
 either = ["dep:either"]
+nan-box-jsvalue = []
 
 # Enables the `Intl` builtin object and bundles a default ICU4X data provider.
 # Prefer this over `intl` if you just want to enable `Intl` without dealing with the