Expose new c-api functions for configuring relaxed SIMD

[martindevans]

This allows the new relaxed SIMD features (introduced in #5892) to be configured through the C API:

Added:

• wasmtime_config_wasm_relaxed_simd_set(bool)
• wasmtime_config_wasm_relaxed_simd_deterministic_set(bool)

[alexcrichton]

Thanks! Can you also add C header API definitions for these new functions?

[github-actions]

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[github-actions]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• If you added a new Config method, you wrote extensive documentation for
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  well.
  
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• If you added a new Config method, or modified an existing one, you
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[martindevans]

I don't think the suggestions by the github-actions bot apply here, since I'm just exposing a config option that already existed?