Replace the support of <code> elements by <script>

The `<script>` element is better than `<code>` element, because:

- no need to remove the element to avoid side effects
- this element is ignored by the browser, so no execution, no rendering, no layout reflow, no pollution of the view (when uilang.js is not executed because of an exception made by an other script)
- This element is ignored by search engines
-  we can have many `<script type="text/uilang">` elements in the page: this is useful when a page is built from several files that have their own uilang instructions
- It respects the HTML semantic: `<code>` is to display code, not to execute code
- uilang can be extended by supporting external uilang files with `<script type="text/uilang" src="file.uilang"/>` (having such features with `<code>` is ugly...)