update tsc target

[ivanwonder]

@LabhanshAgrawal https://github.com/zeit/hyper/blob/802d6064410b86909b4253943e1c4aee9dd4a679/tsconfig.base.json#L20

https://github.com/microsoft/vscode/blob/f4fc810feaed71661d60b5e41e1ce0d39f3e6747/src/tsconfig.json#L8
It's better to update tsc target to es2017/es2018 because the electron is the latest version and no need to be compatible with older browser.
worth trying and testing.

• I am on the latest Hyper.app version
• I have searched the issues of this repo and believe that this is not a duplicate

• OS version and name:
• Hyper.app version:
• Link of a Gist with the contents of your .hyper.js:
• Relevant information from devtools (CMD+ALT+I on macOS, CTRL+SHIFT+I elsewhere):
• The issue is reproducible in vanilla Hyper.app:

Issue

[LabhanshAgrawal]

I tried setting ES2017 and didn't face any issues. I think we can go ahead and use it.