Update eslint to 9.34.0

This requires a switch to the configuration file format described at
https://eslint.org/docs/latest/use/configure/configuration-files. I used
the automatic tool to generate the new file, with some simplifications
around defaults.

- Removed no-cond-assign override, it is no longer needed.
- Fixed `catch (e)` where `e` is not used. This seems a little pedantic
  to me, but seems like a relatively easy fix to just remove it, and I
  believe the syntax without the variable has been supported for quite
  some time. Alternatively it could also be `_e`, or explicitly allowed.
- Added `--no-warn-ignored` to the script since it was very noisy.

eslint.config.mjs

@@ -0,0 +1,72 @@
+import { defineConfig, globalIgnores } from "eslint/config";
+
+export default defineConfig([
+    globalIgnores(["**/**min.js", "**/highlight.js", "**/playground_editor/*"]),
+    {
+        rules: {
+            indent: ["error", 4],
+            "linebreak-style": ["error", "unix"],
+            quotes: ["error", "single"],
+            semi: ["error", "always"],
+
+            "brace-style": ["error", "1tbs", {
+                allowSingleLine: false,
+            }],
+
+            curly: "error",
+            "no-trailing-spaces": "error",
+            "no-multi-spaces": "error",
+
+            "keyword-spacing": ["error", {
+                before: true,
+                after: true,
+            }],
+
+            "comma-spacing": ["error", {
+                before: false,
+                after: true,
+            }],
+
+            "arrow-spacing": ["error", {
+                before: true,
+                after: true,
+            }],
+
+            "key-spacing": ["error", {
+                beforeColon: false,
+                afterColon: true,
+                mode: "strict",
+            }],
+
+            "func-call-spacing": ["error", "never"],
+            "space-infix-ops": "error",
+            "space-before-function-paren": ["error", "never"],
+            "space-before-blocks": "error",
+
+            "no-console": ["error", {
+                allow: ["warn", "error"],
+            }],
+
+            "comma-dangle": ["error", "always-multiline"],
+            "comma-style": ["error", "last"],
+
+            "max-len": ["error", {
+                code: 100,
+                tabWidth: 2,
+            }],
+
+            "eol-last": ["error", "always"],
+            "no-extra-parens": "error",
+            "arrow-parens": ["error", "as-needed"],
+
+            "no-unused-vars": ["error", {
+                argsIgnorePattern: "^_",
+                varsIgnorePattern: "^_",
+            }],
+
+            "prefer-const": ["error"],
+            "no-var": "error",
+            eqeqeq: "error",
+        },
+    },
+]);