SINGLE_FILE and pthread incompatibility

According to https://github.com/emscripten-core/emscripten/pull/12833#issue-746884561 which says the following:

> SINGLE_FILE=1 is not supported with Wasm Workers (like it is not supported with pthreads)

I'm unable to find other specific documentation for pthreads not supporting SINGLE_FILE, but locally I can confirm that removing SINGLE_FILE does indeed fix the threading issue I'm running into (where the thread does not get executed with `detach`).

What is the specific reason(s) that SINGLE_FILE does not work with pthreads? From what I understand about SINGLE_FILE, it adds the contents of the .js and .wasm files (as a Base64 string) into the HTML file as opposed to reading the wasm contents via locateFile - clearly there is something else that I'm missing.

Start by reading the linked Emscripten pull request and comparing the reported pthreads behavior with and without SINGLE_FILE=1. Trace how SINGLE_FILE, locateFile, and Wasm Workers interact in the relevant Emscripten entry points. Done means documenting the specific reasons SINGLE_FILE is incompatible with pthreads and clarifying the supported configuration.

Written by the indexing model from the issue text.