"Browser Usage" in Readme probably wrong due to synchronous webassembly compilation

We specifically compile ``soljson.js`` with ``WASM_ASYNC_COMPILATION=0`` - there was a reason for this, but I don't remember right now.
However, I think at least chromium-based browsers won't allow synchronous compilation from the main thread. This effectively means that loading the compiler *has* to be done from web workers right now (at least if it's supposed to work in all browsers) and the "Browser Usage" section in ``README.md`` is outdated.

So we need to check if we can switch to asynchronous compilation in solidity, investigate if and what kind of problems synchronous compilation entails and adjust the section in the readme accordingly.

Start with the Browser Usage section in README.md and the soljson.js build setting WASM_ASYNC_COMPILATION=0. Check whether synchronous compilation works in Chromium-based browsers on the main thread and whether asynchronous compilation can be enabled; done means the supported loading approach is established and the README accurately reflects it.

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