Support for watch mode in 3.0

[jtpio]

Running in a new environment with the latest JupyterLab 3.0.0rc1, it looks like the watch mode is making the server hangs on startup.

To reproduce:

• create a new environment
• python -m pip install --pre jupyterlab
• jupyter lab --no-browser --watch

This results in the LapApp not logging the server url:

Entrypoint main 406 KiB = vendors-node_modules_whatwg-fetch_fetch_js.f6327745ab1376187dec.js 43.7 KiB main.2deb5ac19ce4adc2793f.js 363 KiB
orphan modules 210 KiB [orphan] 206 modules
runtime modules 140 KiB 14 modules
modules by path ./node_modules/ 13.1 MiB
  cacheable modules 13.1 MiB 998 modules
  3 modules
consume-shared-module modules 2.95 KiB
  modules by path consume shared module (default) @jupyterlab/ 1.93 KiB 47 modules
  modules by path consume shared module (default) @lumino/ 504 bytes 12 modules
  13 modules
provide-module modules 2.42 KiB
  modules by path provide shared module (default) @jupyterlab/ 1.85 KiB 45 modules
  modules by path provide shared module (default) @lumino/ 504 bytes 12 modules
  2 modules
modules by path ./build/ 19.2 KiB
  modules by path ./build/*.js 14.2 KiB 2 modules
  modules by path ./build/*.css 5.06 KiB 2 modules
modules by path  99 bytes 3 modules
./publicpath.js 1.38 KiB [built] [code generated]
webpack 5.0.0-rc.0 compiled successfully in 34155 ms

asset index.css 15.7 KiB [compared for emit] (name: index)
asset index.js 0 bytes [compared for emit] [minimized] (name: index)
Entrypoint index 15.7 KiB = index.css 15.7 KiB index.js 0 bytes
runtime modules 0 bytes 1 module
css modules 15.7 KiB
  css ./node_modules/css-loader/dist/cjs.js!./node_modules/@jupyterlab/theme-dark-extension/style/variables.css 15.3 KiB [built] [code generated]
  css ./node_modules/css-loader/dist/cjs.js!./node_modules/@jupyterlab/theme-dark-extension/style/index.css 472 bytes [built] [code generated]
./node_modules/@jupyterlab/theme-dark-extension/style/index.css 39 bytes [built] [code generated]
webpack 5.0.0-rc.0 compiled successfully in 12405 ms

asset index.css 14.7 KiB [compared for emit] (name: index)
asset index.js 0 bytes [compared for emit] [minimized] (name: index)
Entrypoint index 14.7 KiB = index.css 14.7 KiB index.js 0 bytes
runtime modules 0 bytes 1 module
css modules 14.7 KiB
  css ./node_modules/css-loader/dist/cjs.js!./node_modules/@jupyterlab/theme-light-extension/style/variables.css 14.2 KiB [built] [code generated]
  css ./node_modules/css-loader/dist/cjs.js!./node_modules/@jupyterlab/theme-light-extension/style/index.css 472 bytes [built] [code generated]
./node_modules/@jupyterlab/theme-light-extension/style/index.css 39 bytes [built] [code generated]
webpack 5.0.0-rc.0 compiled successfully in 12401 ms

[jtpio]

We probably still want to support running lab with jupyter lab --watch for 3.0, since some extension authors might still want to keep their previous workflows or bundle their extensions.