JetBrains: Add basic TS pipeline

Created by: philipp-spiess

Part of #34366 (closed) Part of #34186 (closed)

This PR sets up a rudimentary TypeScript build process that creates artifacts that are picked up inside our web view.

The setup code is mostly taken from our VSCode extension and I'll update it to use esbuild once https://github.com/sourcegraph/sourcegraph/pull/34010 is merged.

Test plan

Manually verified that the code is loaded:

The build runs:

α jetbrains (ps/jetbrains-ts-pipeline) yarn build
yarn run v1.22.17
$ yarn task:gulp webpack
$ cross-env NODE_OPTIONS="--max_old_space_size=8192" gulp webpack
[16:14:25] Using gulpfile ~/dev/sourcegraph/client/jetbrains/gulpfile.js
[16:14:25] Starting 'webpack'...
{
  search: '/Users/philipp/dev/sourcegraph/client/jetbrains/webview/search/index.tsx',
  style: '/Users/philipp/dev/sourcegraph/client/jetbrains/webview/index.scss'
}
[16:14:30] webviews:
  webviews compiled in 4511 ms
[16:14:30] Finished 'webpack' after 4.68 s
✨  Done in 6.44s.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.