For large files it's better to have a viewport because
then you can view both the form as well as the part of
the output you are interested in at the same time.

* "Try out Pygments!" is more descriptive than "Try it out!"
* Moved the description of the implementation to the end.
* Added a <noscript> tag suggesting `pip install` and pygmentize.
* Added a <label> for the textarea.
* Disable spell checking for the textarea.
* Reduced the scope of the reset button to the file input.
* Added autofocus for the language select.

Previously the form could only be filled out after Pyodide finished
loading (since the <select> options were generated via Pyodide).

This commit changes this, so that the <select> options are already
in the static HTML. The "Loading Python..." form overlay is removed,
instead just the Highlight button is disabled, letting users fill
out the form while Pyodide is loading.

When a ?code query parameter is given a "Loading Python..." message
is displayed below the form, since in that case users probably want
to wait for the highlighted code (instead of filling out the form).