Fix imports of @bokeh/bokehjs

[ianthomas23]

Fix the imports of @bokeh/bokehjs so we don't need any path workarounds in tsconfig.json or webpack.config.ts. The form used here is:

import * as Bokeh from "@bokeh/bokehjs";

const source = new Bokeh.ColumnDataSource(...);
const plot = Bokeh.Plotting.figure(...);
const button = new Bokeh.Widgets.Button(...);
Bokeh.Plotting.show(...);

An alternative more explicit approach is:

import { ColumnDataSource, Plotting, Widgets } from "@bokeh/bokehjs";`

const source = newColumnDataSource(...);
const plot = Plotting.figure(...);
const button = new Widgets.Button(...);
Plotting.show(...);

I prefer the latter but the former is consistent with the approach used in the existing Bokeh docs at https://docs.bokeh.org/en/latest/docs/user_guide/advanced/bokehjs.html, so we will use that.