Some custom WASI SDK fixes

[jeffcharles]

Some improvements for custom WASI SDK handling.

1. Introduce the QUICKJS_WASM_SYS_WASI_SDK_MAJOR_VERSION and QUICKJS_WASM_SYS_WASI_SDK_MINOR_VERSION environment variables for customizing the versions of the WASI SDK to use without having to recompile the build script for quickjs-wasm-sys. This is helpful for quickly experimenting with using different versions of the WASI SDK.
2. Trigger a recompilation if any of the values for QUICKJS_WASM_SYS_WASI_SDK_PATH, QUICKJS_WASM_SYS_WASI_SDK_MAJOR_VERSION, or QUICKJS_WASM_SYS_WASI_SDK_MINOR_VERSION change. This fixes a bug where changes to the WASI SDK may not have triggered a recompilation as expected.
3. Remove the Makefile target for download-wasi-sdk and associated shell script. This target and shell script should no longer be necessary given the build script and the new environment variables for quickly experimenting with new SDK versions.

I'm opting to do a minor version bump for quickjs-wasm-sys given the new build time environment variables would technically count as new features I think.