Add ability to auto update Insiders build of extension

Others have done this https://github.com/Microsoft/vscode/issues/15756#issuecomment-425287642

Updating label to code health + enhancement. I.e. even if not exposed to end users, it would be good for in-house use. Much like out of band LS installs.

[DonJayamanne]

Note: This is not for extension authors. That python file builds the extension. This is for users of the developer build of the extension, i.e. dog fooding. VSC doesn’t provide a pre-release channel of extensions, and that’s why we host our extension on a blob store, and users need to install it manually.

The benefit, is:

• We’ll have a regular set of users who are constantly testing/using the dev version of the extension.
• We have the potential to get feedback before the release.

Either way, this would be great for in house developers.

UX (similar to out of band installs of LS):

• Download and install extension daily, weekly
• Manual download and install of the extension (via some command)

[ericsnowcurrently]

FWIW, Don manually double-checked about the prompt. It worked as expected. I must have been prompted a while back and VS Code still remembered.