webassembly: Make mp_js_process_char asynchronous.

This may also call the garbage collector.
Signed-off-by: NEli Bierman <eli@elib.dev>

ports/webassembly/README.md

@@ -121,7 +121,7 @@ Initialize MicroPython repl. Must be called before entering characters into
 the repl.
 
 ```
-mp_js_process_char(char)
+await mp_js_process_char(char)
 ```
 
 Input character into MicroPython repl. `char` must be of type `number`. This

ports/webassembly/wrapper.js

@@ -31,7 +31,7 @@ var mainProgram = function()
   mp_js_init = Module.cwrap('mp_js_init', 'null', ['number']);
   mp_js_do_str = Module.cwrap('mp_js_do_str', 'number', ['string'], {async: true});
   mp_js_init_repl = Module.cwrap('mp_js_init_repl', 'null', ['null']);
-  mp_js_process_char = Module.cwrap('mp_js_process_char', 'number', ['number']);
+  mp_js_process_char = Module.cwrap('mp_js_process_char', 'number', ['number'], {async: true});
 
   MP_JS_EPOCH = Date.now();
 
@@ -69,9 +69,11 @@ var mainProgram = function()
           process.stdin.setRawMode(true);
           process.stdin.on('data', function (data) {
               for (var i = 0; i < data.length; i++) {
-                  if (mp_js_process_char(data[i])) {
-                      process.exit()
-                  }
+                  mp_js_process_char(data[i]).then(result => {
+                      if (result) {
+                          process.exit()
+                      }
+                  })
               }
           });
       } else {