Add emscripten-forge build support for WebAssembly deployment

[Copilot]

This PR adds support for building vbjax packages for WebAssembly environments using emscripten-forge, enabling deployment in web-based Jupyter environments like JupyterLite with xeus-python kernels.

Changes Made

• environment-wasm-build.yml: Conda environment specification for emscripten builds with all necessary dependencies
• .github/workflows/emscripten-forge-build.yml: GitHub Action workflow that automatically builds vbjax for the emscripten-wasm32 platform using rattler-build
• WASM_BUILD.md: Documentation explaining the WebAssembly build process, current limitations, and usage instructions
• README.md: Added WebAssembly support section with clear limitation warnings

Technical Implementation

The workflow uses rattler-build to create conda packages for the emscripten-wasm32 platform. It includes:

• Smart version extraction from _version.py or Git tags
• Proper handling of dependencies (numpy, scipy) while acknowledging JAX limitations
• Artifact uploading for distribution
• Minimal testing that avoids JAX import issues

Current Limitations

⚠️ Important: JAX and JAXlib do not currently have full WebAssembly support in emscripten environments. This build:

• ✅ Creates a valid conda package structure
• ✅ Can be distributed via emscripten-forge
• ✅ Can be installed in xeus-python kernels
• ❌ Has limited functionality until JAX WebAssembly support improves

Future Benefits

As JAX WebAssembly support matures, this infrastructure will enable:

• Full vbjax functionality in web browsers
• Integration with JupyterLite deployments
• Browser-based neural mass modeling and simulation

The build configuration is designed to be easily updated when JAX WebAssembly compatibility improves.

Fixes #88.

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