Re-export HEAP* views on Module for embedders (#76)

Recent Emscripten (>=3.1.x) makes the HEAP{,U}{8,16,32} TypedArray
views module-local by default to help tree-shaking. The bundled
simple.js (and any other code that embeds the Binjgb factory)
reaches into module.HEAP8.buffer / module.HEAPU8 / etc. via
makeWasmBuffer; with the new default those accesses are undefined
and the first Emulator construction throws
"Cannot read properties of undefined (reading 'buffer')" before
the ROM even loads.

Add an explicit EXPORTED_RUNTIME_METHODS list so the wasm build
re-attaches these views to the Module object the way pre-3.1.x
Emscripten did by default. Also gitignore the `emscripten`
symlink that contributors create at the repo root per the README
build instructions.

docs/binjgb.{js,wasm} regenerated via `make demo` under
Emscripten 5.0.7.

Author: kwigbo

.gitignore

@@ -1,5 +1,6 @@
 /bin
 /out
+/emscripten
 /test/mooneye-gb
 /test/mooneye-gb-wp
 /third_party/wla-dx

CMakeLists.txt

@@ -161,6 +161,7 @@ else (EMSCRIPTEN)
 
   set(LINK_FLAGS
     -s EXPORTED_FUNCTIONS=\"@${EXPORTED_JSON}\"
+    -s EXPORTED_RUNTIME_METHODS=\"['HEAP8','HEAPU8','HEAP16','HEAPU16','HEAP32','HEAPU32','HEAPF32','HEAPF64']\"
     -s MALLOC=emmalloc
     -s ASSERTIONS=0
     -s ENVIRONMENT=web