`use` statement, refactor, fix

Changelog ✨

• use statement - keep your component exemplars in variables and work with it like with objects ✌

  var comp = use:
    component MyComponent(myArg = 100):
      "component slot"
      tDiv:
        ...
  echo comp.myArg.val
  buildHtml:
    component comp
    component MyComponent(myArg = 101)

• elem statement (shortcut for document.getElementById) ✨

  <input id="myInput">

  echo elem(myInput).value

• Built-in UI components 🎴 (compile with -d:enableUi)
  Docs
  Example:

  buildHtml:
    component Button:
      "Click me!"
    component Input(placeholder = "Edit text ...", label = "Edit text ...")

• events in event handlers 🛠

  buildHtml:
    tInput:
      @input(ev):  # `ev` is just param name. It may be anything. Works with any built-in web event
        echo ev.data

• Callable states available 🔥
  Before:

  self.state.val()()

  Now:

  self.state()

• Fix CSS into component style 🌿