Reverts: #56206
Initiated by: jonahwilliams
Reason for reverting: failing to roll into framework: flutter/flutter#157919 
Original PR Author: eyebrowsoffire

Reviewed By: {harryterkelsen}

This change reverts the following previous change:
This PR creates a single-threaded version of the skwasm renderer, appropriate for non-crossOriginIsolated browsing contexts.
* The single threaded renderer is essentially the same as the multi-threaded renderer, except instead of spawning a web worker and posting messages to it, it simply schedules microtasks on the main thread in their place.
* The new renderer is vended as `skwasm_st.js` and `skwasm_st.wasm` in the same location as multithreaded skwasm. In order to properly build and function, we needed some fixes I put into emscripten that landed in version 3.1.70. That version also changed some behavior that required a few fixes to the CanvasKit build files.
* The skwasm loader in flutter.js has been modified to use the skwasm_st variants when encountering a non-crossOriginIsolated context but a browser and configuration that otherwise would allow the use of skwasm. I also added a new `forceSingleThreadedSkwasm` option to the flutter configuration so that we can override this behavior, especially so that we can accurately benchmark the single threaded renderer in a crossOriginIsolated environment.
* I also consolidated a bunch of our shards that run tests to just have one per browser/platform combination, so four total. This will address flutter/flutter#124682

Reverts: flutter/engine#56206
Initiated by: jonahwilliams
Reason for reverting: failing to roll into framework: flutter#157919 
Original PR Author: eyebrowsoffire

Reviewed By: {harryterkelsen}

This change reverts the following previous change:
This PR creates a single-threaded version of the skwasm renderer, appropriate for non-crossOriginIsolated browsing contexts.
* The single threaded renderer is essentially the same as the multi-threaded renderer, except instead of spawning a web worker and posting messages to it, it simply schedules microtasks on the main thread in their place.
* The new renderer is vended as `skwasm_st.js` and `skwasm_st.wasm` in the same location as multithreaded skwasm. In order to properly build and function, we needed some fixes I put into emscripten that landed in version 3.1.70. That version also changed some behavior that required a few fixes to the CanvasKit build files.
* The skwasm loader in flutter.js has been modified to use the skwasm_st variants when encountering a non-crossOriginIsolated context but a browser and configuration that otherwise would allow the use of skwasm. I also added a new `forceSingleThreadedSkwasm` option to the flutter configuration so that we can override this behavior, especially so that we can accurately benchmark the single threaded renderer in a crossOriginIsolated environment.
* I also consolidated a bunch of our shards that run tests to just have one per browser/platform combination, so four total. This will address flutter#124682