feature/sync preview to editor

the first commit adds code (largely copied and adapted from anru/rsted@f46ff7b665e9abc3cddaad358a0ce0e0ef8bbdce) to keep the visible area of the preview div in sync with text being edited in the editor.

the second commit is just a version bump with some suffix to avoid clashes with upstream versioning, including re-build of js and css via build.js

i have only tested this with a ~500 lines/~5K words text without images and on wideish screens (1920px) so far, only very briefly on narrower screens.

the sync logic as per anru/rsted@f46ff7b665e9abc3cddaad358a0ce0e0ef8bbdce is rather simple so it might be subptimal for complex rendered layouts, but seems to be working fairly well for me so far.