Feature request: using WASM exceptions.

[Trafo]

At the moment Botan is not using the WASM exceptions:

lang_flags "-s DISABLE_EXCEPTION_CATCHING=0 -std=c++20 -D_REENTRANT"
lang_binary_linker_flags "-s ALLOW_MEMORY_GROWTH=1 -s WASM=1 -s NO_DISABLE_EXCEPTION_CATCHING"

(in src/build-data/cc/emcc.txt) Can we make it configurable or directly switch to use WASM exceptions, which are from my point of view, much better?:

lang_flags "-fwasm-exceptions -std=c++20 -D_REENTRANT"
lang_binary_linker_flags "-s ALLOW_MEMORY_GROWTH=1 -s WASM=1 -fwasm-exceptions"

[randombit]

@reneme it looks like you added the Emscripten build; wdyt?

[reneme]

The patch you propose looks reasonable to me. Please feel free to create a pull request for that, @Trafo.