By using a persistent WebGL context for reach (small) preview, we hit
the browser imposed limits on the number of GL contexts quickly. This
change add support to pt out of the automatic enabling of the preview
via no-auto-preview, that's selected for recipes.
Highlight all Slint code and don't do the preview on snippets where it
doesn't make sense.
Some snippets, such as the bare statement or expression snippets, are
now highlighted, but they are continued to be excluded from the doctest.
