Websites: Code mode keeps up with typing in large files

In the Websites editor's Code mode, each keystroke used to recolour the whole file and, after a short pause, reload the preview, which made typing in a long layout or stylesheet lag. Colouring now works line by line, so only the lines that changed are redrawn, and the preview beside the code is rebuilt about a second after typing stops instead of in every short pause. The colour swatches and the Colours list also update once typing stops rather than on every keystroke. Nothing about what the colouring shows has changed.