chore: move to eslint v9 (#7041)

- Upgrades eslint from v8 to v9
- Upgrades all other eslint packages. We will have to do a new
full-project lint, as new rules have been added
- Upgrades husky from v8 to v9
- Upgrades lint-staged from v14 to v15
- Moves the old .eslintrc.cjs file format to the new eslint.config.js
flat file format.

Previously, we were very specific regarding which rules are applied to
which files. Now that `extends` is no longer a thing, I have to use
deepMerge & imports instead.

This is rather uncommon and is not a documented pattern - e.g.
typescript-eslint docs want us to add the default typescript-eslint
rules to the top-level & then disable it in files using the
disable-typechecked config.

However, I hate this opt-out approach. The way I did it here adds a lot
of clarity as to which rules are applied to which files, and is pretty
easy to read. Much less black magic

## .eslintignore

These files are no longer supported (see
https://eslint.org/docs/latest/use/configure/migration-guide#ignoring-files).
I moved the entries to the ignores property in the eslint config. => one
less file in each package folder!

packages/eslint-plugin/index.mjs

@@ -0,0 +1,37 @@
+import noJsxImportStatements from './customRules/no-jsx-import-statements.js'
+import noNonRetryableAssertions from './customRules/no-non-retryable-assertions.js'
+import noRelativeMonorepoImports from './customRules/no-relative-monorepo-imports.js'
+import noImportsFromExportsDir from './customRules/no-imports-from-exports-dir.js'
+import noFlakyAssertions from './customRules/no-flaky-assertions.js'
+
+
+/**
+ * @type {import('eslint').ESLint.Plugin}
+ */
+const index = {
+  rules: {
+    'no-jsx-import-statements': noJsxImportStatements,
+    'no-non-retryable-assertions': noNonRetryableAssertions,
+    'no-relative-monorepo-imports': noRelativeMonorepoImports,
+    'no-imports-from-exports-dir': noImportsFromExportsDir,
+    'no-flaky-assertions': noFlakyAssertions,
+    'no-wait-function': {
+      create: function (context) {
+        return {
+          CallExpression(node) {
+            // Check if the function being called is named "wait"
+            if (node.callee.name === 'wait') {
+              context.report({
+                node,
+                message:
+                  'Usage of "wait" function is discouraged as it\'s flaky. Proper assertions should be used instead.',
+              })
+            }
+          },
+        }
+      },
+    },
+  },
+}
+
+export default index