🏗 Update @percy/percy-puppeteer to v1.x

[danielrozenberg]

Fixes #24869

This PR updates the @percy/percy-puppeteer dependency from v0.4.0 to v1.0.8, and also:

Major:

• Executes the @percy/agent binary in a subprocess of the gulp task (the agent is required by the updated dependency)
• Removes the special-case for Travis logs (running series of dots) and display each test start/end on Travis
  • This was done because the new Percy agent is verbose on its own, and doesn't have an easy way to silence it. Since the visual diff tests are now running on their own separate shard, this doesn't spam the logs
  • To display verbose logs locally, pass --verbose to the task

Minor:

• Removes the custom assets loader, since assets are handled by the updated dependency
• Removes unnecessary .gitkeep file for a no-longer blank directory
• Replaced default shell for spawned processes on Unix from /bin/sh to /bin/bash per Can't kill child process travis-ci/travis-ci#704 (comment)

[rsimha]

Really excited to see this happen! Some initial comments below.

[rsimha]

Checked out this branch on a Macbook and sanity tested commands like gulp dist and gulp check-types, both of which use execScriptAsync under the covers. Everything looks good so far. Thanks for doing this!