Opt insiders users into beta language server by default

[brettcannon]

[karrtikr]

Prescribed solution

We want the default channel to be "beta" when user is using "insiders".

• Check https://github.com/Microsoft/vscode-python/blob/2c7a4616e0c9c9ae14c67c501d3a2f2d5793a6da/src/client/activation/languageServer/languageServerPackageService.ts#L64

• If the user is using default configuration for setting python.analysis.downloadChannel, and python.insidersChannel is "weekly" or "daily", we return "beta" instead of returning settings.analysis.downloadChannel.