Rename `emscripten/heap.h` and functions like `emscripten_get_heap_size`

For historical reasons there are some places in emscripten where we use the term `heap` to refer to the entire linear memory. Where is the more common usage of the term heap is to refer to just the malloc-controlled section (not the shadow stack or the static data parts).

See #15793 for an example of the confusion this causes.

I guess its probably to hard to rename HEAPU8 and friends!