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/richtext-slate/eslint.config.js

@@ -0,0 +1,20 @@
+import { rootEslintConfig, rootParserOptions } from '../../eslint.config.js'
+
+/** @typedef {import('eslint').Linter.FlatConfig} */
+let FlatConfig
+
+/** @type {FlatConfig[]} */
+export const index = [
+  ...rootEslintConfig,
+  {
+    languageOptions: {
+      parserOptions: {
+        project: './tsconfig.json',
+        tsconfigDirName: import.meta.dirname,
+        ...rootParserOptions,
+      },
+    },
+  },
+]
+
+export default index