Run markdown parser and code highlighter in Worker

[Cheng]

Currently the most expensive operations in Chie are markdown parsing and code highlighting, they are directly responsible for slow startup and lagging, they are also responsible for a large part of memory usage (30%~50%).

By running them in a Worker and making rendering code async, it should be possible to make the interface much more responsive with long conversations. Also by dynamically creating and destroying the Worker, we should be able to free the memory taken by the parsers when the app is inactive.