Design API to access tslab utilities from other libraties

[yunabe]

From notebooks, users can use "tslab" module to access tslab utilities.

import * as tslab from "tslab";
tslab.display.jpeg(img);

But this does not work well when other libraries want to use tslab library because

1. Libraries do not want to depend on tslab because they are not always used under tslab.
2. If the version of tslab main routine and the version of tslab which other libraries depend on are different, two+ tslab modules are installed separately and non-main tslab library does not work properly.

Thus, we need to design a proper API for other libraries so that they can implement special handlings when they are used under tslab.

[dbuezas]

This would be useful for the https://github.com/dbuezas/tslab-plotly lib