Abandon the .mjs naming scheme

Browsers refuse to load a JS module if the mimetype is wrong, and even
Python's http.server doesn't recognize this extension yet.

js/main-editor.js

@@ -1,4 +1,4 @@
-import * as fflate from './vendor/fflate.mjs';
+import * as fflate from './vendor/fflate.js';
 
 import { DIRECTIONS, LAYERS, TICS_PER_SECOND } from './defs.js';
 import { TILES_WITH_PROPS } from './editor-tile-overlays.js';

js/main.js

@@ -1,6 +1,6 @@
 // TODO bugs and quirks i'm aware of:
 // - steam: if a player character starts on a force floor they won't be able to make any voluntary movements until they are no longer on a force floor
-import * as fflate from './vendor/fflate.mjs';
+import * as fflate from './vendor/fflate.js';
 
 import { COMPAT_FLAGS, COMPAT_RULESET_LABELS, COMPAT_RULESET_ORDER, DIRECTIONS, INPUT_BITS, TICS_PER_SECOND, compat_flags_for_ruleset } from './defs.js';
 import * as c2g from './format-c2g.js';

js/util.js

@@ -1,4 +1,4 @@
-import * as fflate from './vendor/fflate.mjs';
+import * as fflate from './vendor/fflate.js';
 
 // Base class for custom errors
 export class LLError extends Error {}