Update Seti to support Notebook icons

[rebornix]

Rerun update-icon-theme.js to get the latest Seti icons. Current one doesn't pick the notebook icon for ipynb (it shows as JSON) but once it's updated, it works as expected.

[rebornix]

@aeschli can you please help take a look at this PR?

[aeschli]

You need to first run npm run prepublishOnly in the seti repo to update the font and font character table (seti.less).
The run npm run update in the extensions/theme-seti directory.

I pushed the update.

[rebornix]

Thanks @aeschli !