add javascript support

[wilsonwang371]

This pull request introduces initial infrastructure and documentation for Rust-based WASM samples using the Boa JavaScript engine, and adds a detailed SDK design proposal (in both English and Chinese). The changes enable building Rust WASM samples alongside existing C samples, clarify sample build outputs, and lay out the architecture and API design for a future Rust SDK that integrates Spear hostcalls into a JS-friendly environment.

Build system enhancements:

• Added support for building Rust-based WASM samples in the Makefile, including logic to detect and build Rust samples with Cargo, copy resulting WASM artifacts to samples/build/rust/, and provide user feedback if prerequisites are missing.
• Introduced new Makefile variables for Rust sample directories, build output paths, sample names, and a flag to enable/disable Rust sample builds.

Documentation updates:

• Updated both English (README.md) and Chinese (README.zh.md) READMEs to document that sample build artifacts are now output to samples/build/ (for C) and samples/build/rust/ (for Rust). [1] [2]
• Added comprehensive English and Chinese design documents (docs/rust-sdk-boa-wasm-design-en.md and docs/rust-sdk-boa-wasm-design-zh.md) detailing the proposed Rust SDK architecture, goals, API design, integration with Boa, tool calling, audio streaming, and future enhancements. [1] [2]