Support webviews without internet

[sguillia]

I can't seem to open a webview without a working internet connection.

VS Code is doing request on vscode-webview-test.com, which fail without internet.

I cannot find that endpoint in your documentation, only in source code

Could it be possible to

• document why this endpoint is needed
• make webviews work without it
• allow users to opt-out of this call

Would be wonderful! Thanks a lot

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Steps to Reproduce:

1. Open this project
2. Disable your internet connection
3. Run command "Start cat coding session"

Version: 1.44.0-insider
Commit: 13b38d2
Date: 2020-03-30T08:08:20.753Z (2 wks ago)
Electron: 7.1.11
Chrome: 78.0.3904.130
Node.js: 12.8.1
V8: 7.8.279.23-electron.0
OS: Linux x64 4.15.0-91-generic

Reproduces with all extensions disabled

[sguillia]

I forgot I had this in my settings "webview.experimental.useExternalEndpoint": true in order to make webviews work.

Apologies.

Related to #83188