Convert backend to ESM

- About 5 years overdue
- Remove eslint, use bomejs instead

backend/.eslintrc.json

@@ -1,73 +0,0 @@
-{
-	"env": {
-		"node": true,
-		"es6": true
-	},
-	"extends": [
-		"eslint:recommended"
-	],
-	"globals": {
-		"Atomics": "readonly",
-		"SharedArrayBuffer": "readonly"
-	},
-	"parserOptions": {
-		"ecmaVersion": 2018,
-		"sourceType": "module"
-	},
-	"plugins": [
-		"align-assignments"
-	],
-	"rules": {
-		"arrow-parens": [
-			"error",
-			"always"
-		],
-		"indent": [
-			"error",
-			"tab"
-		],
-		"linebreak-style": [
-			"error",
-			"unix"
-		],
-		"quotes": [
-			"error",
-			"single"
-		],
-		"semi": [
-			"error",
-			"always"
-		],
-		"key-spacing": [
-			"error",
-			{
-				"align": "value"
-			}
-		],
-		"comma-spacing": [
-			"error",
-			{
-				"before": false,
-				"after": true
-			}
-		],
-		"func-call-spacing": [
-			"error",
-			"never"
-		],
-		"keyword-spacing": [
-			"error",
-			{
-				"before": true
-			}
-		],
-		"no-irregular-whitespace": "error",
-		"no-unused-expressions": 0,
-		"align-assignments/align-assignments": [
-			2,
-			{
-				"requiresOnly": false
-			}
-		]
-	}
-}