[doc] Make sure extension developers compile against ES5

[kittaakos]

Description

Compiling against es6 can cause strange behavior at runtime. See #6740 (comment). If it is a known limitation, we need to "advertise" it better among the extension developers. It took me some time to figure out what was the problem.

If it is a known limitation

Is it?
CC: @akosyakov

Reproduction Steps

OS and Theia version:

Diagnostics:

[akosyakov]

Yes, for now, it would be safer to use es5.

[vince-fugnitto]

Where would be the best place(s) to advertise which target to compile to for extension developers?

[akosyakov]

Honestly, i thought that everyone is using theia extension generator to get started or copy configs from existing extensions. In both cases it should be es5. Such thing can happen only if someone intentionally switched to es6 without giving a thought to it.

[mikael-desharnais]

You have to add "skipLibCheck": true to your tsconfig.json. I had the exact same problem an hour ago, so I am going to close it. Also, make sure you compile against ES5: "target": "es5",.

Sorry but isn't it a bit ugly to skipLibCheck ?