Use external source map in dev mode to avoid dirtying dist js

Author: tfwright

.gitignore

@@ -37,3 +37,6 @@ live_admin-*.tar
 .DS_Store
 
 archives/
+
+# Source maps (generated in dev watch mode)
+dist/js/*.map

assets/esbuild.js

@@ -23,7 +23,7 @@ let opts = {
 }
 if (mode === 'watch') {
   opts = {
-    sourcemap: 'inline',
+    sourcemap: 'external',
     ...opts
   }
 }