TypeScript declarations should generate `MainModule` type name based on user inputs about emscripten

[sbc100]

Since this type is already encapsulated within the module perhaps its OK for the names to collide?

FWIW I reallly don't like MainModule as a name, but that is a different issue.

Presumably it might even be useful to have them all called the same thing if you are working with multiple modules.

If we do decide that unique names within the module are a good idea how about using the -o filename as the default?

e.g. ./emcc -o foo.js would default to EXPORT_NAME or foo?

Also, is this issue a duplicate of #20952?