Change module loading keyword to with

See: tc39/proposal-import-attributes#131

Explanation: https://stackoverflow.com/questions/78876691/syntaxerror-unexpected-identifier-assert-on-json-import-in-node-v22

Author: nstratos

ability-toggle.js

@@ -1,4 +1,4 @@
-import stylesheet from "./ability-toggle.css" assert { type: "css" }
+import stylesheet from "./ability-toggle.css" with { type: "css" }
 
 const template = document.createElement("template")
 template.innerHTML = `

ability-tree.js

@@ -1,4 +1,4 @@
-import stylesheet from "./ability-tree.css" assert { type: "css" }
+import stylesheet from "./ability-tree.css" with { type: "css" }
 
 class AbilityTree extends HTMLElement {
   obtainedAbilities = new Map();