Further improve Qt configure user experience when using JSPI

Configure Options

The current configure API for enabling JSPI is:

configure [...] -device-option QT_EMSCRIPTEN_ASYNCIFY=2 -feature-wasm-exceptions

This can be simplified by adding a "wasm-jspi" feature:

configure [...] -wasm-jspi

Enabling Exceptions

Should "wasm-jspi also enable wasm-exceptions? Yes: QtCore enables exceptions unconditionally, which for the JSPI build should be wasm-exceptions. Since wasm-exceptions need a linker flag we need to enable wasm-exceptions at configure time. (This is unlike the default Emscripten exceptions which requires a compile-flag only)