more options for head_content

[hhaensel]

This PR has two parts:

• accept AbstractStrings and Vectors for head_content
• accept head_content in DEFAULT_LAYOUT

  
  cesium_head = [
  script(src = "https://cesium.com/downloads/cesiumjs/releases/1.113/Build/Cesium/Cesium.js", ),
  link(href = "https://cesium.com/downloads/cesiumjs/releases/1.113/Build/Cesium/Widgets/widgets.css", rel = "stylesheet", )
  ]

route("/") do page(@init, ui, head_content = cesium_head) end

or alternatively

@page("/", ui, Stipple.ReactiveTools.DEFAULT_LAYOUT(head_content = cesium_head))

Note: for the latter version to work, you have to delay a potential init script if it refers to DOM elements until they are visible, typically, when the app is ready. `js_initscript` is a wrapper, that executes code after the webtransport is subscribed.
```julia
cesium_module_text = """
    Cesium.Ion.defaultAccessToken = '$cesium_token';
    const viewer = new Cesium.Viewer('cesiumContainer', {
      terrain: Cesium.Terrain.fromWorldTerrain(),
    });    

    // Fly the camera to San Francisco at the given longitude, latitude, and height.
    viewer.camera.flyTo({
      destination: Cesium.Cartesian3.fromDegrees(-122.4175, 37.655, 400),
      orientation: {
        heading: Cesium.Math.toRadians(0.0),
        pitch: Cesium.Math.toRadians(-15.0),
      }
    });

    // Add Cesium OSM Buildings, a global 3D buildings layer.
    const buildingTileset = await Cesium.createOsmBuildingsAsync();
    viewer.scene.primitives.add(buildingTileset);
"""

cesium_module() = [script(Stipple.js_initscript("""
  newscript = document.createElement('script')
  newscript.text = $(json(cesium_module_text))
  newscript.type = 'module'
  document.head.appendChild(newscript)
"""))]

@deps cesium_module