feat: Add bundled feature

[logan-gatlin]

Hello! I'm working on porting node API's to Rust in workerd. I'd like to use these Rust bindings, but they pull in an additional copy of simdutf. This PR adds a bundled feature which when off, allows for linking against an existing simdutf instead.

The bundled feature is on by default. Any users who have default features off will need to explicitly add the bundled feature, so this is a breaking change.

[Copilot]

Pull request overview

This PR adds an optional bundled Cargo feature to the simdutf crate so that consumers (e.g. the workerd Node API port) can link against an externally provided simdutf library instead of compiling the vendored amalgamation, avoiding a duplicate copy of simdutf. When bundled is enabled (the default), the crate compiles the vendored simdutf.cpp; when disabled, it includes only simdutf.h and relies on SIMDUTF_LIB_DIR/SIMDUTF_LIB_NAME to locate/link an existing library. As noted in the description, this is a breaking change for users who disable default features.

Changes:

• Introduce a bundled feature (default-on) and a links = "simdutf" key in Cargo.toml.
• Guard the vendored #include "simdutf.cpp" behind SIMDUTFRS_BUNDLED, falling back to #include "simdutf.h" (in both the generated simdutfrs.cpp and its codegen source).
• Update build.rs to define SIMDUTFRS_BUNDLED for bundled builds and emit link search/lib directives from SIMDUTF_LIB_DIR/SIMDUTF_LIB_NAME otherwise.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
Cargo.toml	Adds bundled feature (default-on) and links = "simdutf" metadata.
build.rs	Conditionally defines SIMDUTFRS_BUNDLED and wires up external lib search/link env vars; contains a build-script feature-detection bug.
cpp/simdutfrs.cpp	Wraps the vendored simdutf.cpp include in an SIMDUTFRS_BUNDLED guard with a simdutf.h fallback.
codegen/src/bindings.rs	Updates the codegen template to emit the same conditional include block.

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