feat: rewrite to Next.js v13 app directory

Improvements:
- Hide switch theme input (ugly little white square)
- i18n without subpath (e.g: /fr or /en), same url whatever the locale used

cypress/e2e/pages/blog/[slug].cy.ts

@@ -1,7 +1,7 @@
 describe('Page /blog/[slug]', () => {
   it('should displays the first blog post (`hello-world`)', () => {
     cy.visit('/blog/hello-world')
-    cy.get('[data-cy=language-flag-text]').should('not.exist')
+    cy.get('[data-cy=locale-flag-text]').should('not.exist')
     cy.get('h1').should('have.text', '👋 Hello, world!')
     cy.get('.prose a:visible').should('have.attr', 'target', '_blank')
   })