Add support for yarn v2 in "next steps" message

Merge security upgrades from dependabot
Update troubleshooting instructions for webpack caching

What's Changed

• Emclain/yarn berry messages by @emclain in #18
• chore(deps): bump lodash from 4.17.19 to 4.17.21 by @dependabot in #13
• chore(deps): bump hosted-git-info from 2.8.8 to 2.8.9 by @dependabot in #14
• chore(deps): bump glob-parent from 5.1.1 to 5.1.2 by @dependabot in #15
• chore(deps): bump path-parse from 1.0.6 to 1.0.7 by @dependabot in #16
• chore(deps): bump minimist from 1.2.5 to 1.2.6 by @dependabot in #17
• add troubleshooting section to readme by @greyepoxy in #20
• Polyfill Array.prototype.flat() for node < v11 by @emclain in #21

New Contributors

• @emclain made their first contribution in #18

Full Changelog: 1.2.0...v1.3.0