refactor: simplify slotted text-field rendering and styles

[web-padawan]

Some improvements to the current V24 based implementation before V25 preparations:

• Changed to use Lit template for rendering slotted vaadin-text-field
  • This is also how we do it nowadays e.g. in vaadin-login-form etc
• Changed to use SlotStylesMixin for applying styles to that text-field
  • The mixin exists specifically for such cases (managing <style> tags)
• Changed to use order CSS property for placing toggle button at the end
  • Removed hack using MutationObserver and <slot> flex direction
• Changed to set ARIA attributes on the actual slotted <input> element

[web-padawan]

Closing in favor of #13 - let's proceed with a single PR for V25 upgrade.