[WebAssembly] Set -fwasm-exceptions in conan/profiles/release-clang-emscripten-wasm

[rturrado]

Through tools.build:cxxflags. This is not working at the moment.

With this, we could just requires antlr4-cppruntime through conanfile.py, and, if needed, the sources would be downloaded and built with -fwasm-exceptions.

So we wouldn't need to FetchContent it, and build it ourselves. Although this is working at the moment.