Update Binder to use JLab 2.0

[jtpio]

With this configuration, Binder will always point to the master branch and build the install the extension from the repo.

Once 2.0 final it out, postBuild could instead install a released version (if this extension is not merged into core).

To test this branch: https://mybinder.org/v2/gh/jtpio/jupyterlab-shortcutui/binder?urlpath=lab

[jtpio]

Also, the build seems to be failing on Binder and locally with:

$ jlpm run build
yarn run v1.21.1
$ tsc
node_modules/@jupyterlab/ui-components/lib/icon/jlicon.d.ts:1:8 - error TS1259: Module '"/home/jtp/git/jtpio/jupyterlab-shortcutui/node_modules/@types/react/index"' can only be default-imported using the 'esModuleInterop' flag

1 import React from 'react';
         ~~~~~

  node_modules/@types/react/index.d.ts:61:1
    61 export = React;
       ~~~~~~~~~~~~~~~
    This module is declared with using 'export =', and can only be used with a default import when using the 'esModuleInterop' flag.

Found 2 errors.

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

After creating a new environment from scratch:

conda env create
conda activate jupyterlab-shortcutui
jlpm
jlpm run build