Replace `@tinymce/eslint-plugin` with a project-owned ESLint config

[curdopet]

Context

Linting currently relies entirely on @tinymce/eslint-plugin (Tiny Technologies' shareable config). .eslintrc.json extends plugin:@tinymce/standard and this single package provides the whole toolchain — eslint, @typescript-eslint/parser + @typescript-eslint/eslint-plugin, eslint-plugin-import, eslint-plugin-mocha, eslint-plugin-prefer-arrow, plus Tiny's custom rules (e.g. @tinymce/prefer-fun).

Since this is a wrapper for OpenTiny, we want to drop the @tinymce/* dev tooling and own our lint configuration.

Motivation

• Remove the @tinymce/* branding/dependency from dev tooling.
• Unblock a newer @typescript-eslint. @tinymce/eslint-plugin@2.3.1 pins an older @typescript-eslint that predates our TypeScript version — this is the source of the "TypeScript version not officially supported" warning. Owning the config lets us use a current version and clear the warning.

Scope

• Add direct devDependencies: eslint, @typescript-eslint/parser, @typescript-eslint/eslint-plugin, eslint-plugin-import (and React/react-hooks plugins if desired).
• Write a new ESLint config (flat eslint.config.js or legacy .eslintrc) based on eslint:recommended + @typescript-eslint/recommended.
• Port the intentional rule overrides currently in .eslintrc.json (e.g. the @typescript-eslint/no-unsafe-argument off; drop the @tinymce/prefer-fun rule, which won't exist).
• Remove @tinymce/eslint-plugin from package.json and refresh yarn.lock.
• Fix any new lint violations surfaced by the different ruleset.
• Ensure the lint script and CI still work.