Add help information to the extension search view

[cmoog]

Show a yellow or red call to action in the extension search view informing the user of all alternative extension installation methods.

[jsjoeio]

@cmoog

I read the discussion around your previous PR that attempted to solve this: #2185

Are we okay adding a similar message to the UI like you were planning or do you want to hold off until our extension marketplace is open source (if we move in that direction)?

[jsjoeio]

Spoke to @cmoog over Slack:

So it’s probably the case that, at this point, anything in the UI would be a big improvement from the current state. Possibly a link to open-vsix or even just a warning that the extensions are not official with a link to the code-server FAQ
But fair warning that any reference to using the MSFT marketplace assets is probably against TOS.

I'm going to brainstorm some ideas and then I'll work on this.

[jsjoeio]

Okay here's what I came up with.

Message to user:

Warning: These extensions are not official. Find open-source extensions here. See docs.

I should be able to open a PR for this next week.

[jsjoeio]

Closing via #2501