Make config synchronously available to extensions

Created by: sqs

Extensions currently must wait until after activate() is called to get the configuration. The configuration is not synchronously available. So a lot of extensions have workarounds like:

export async function activate(): Promise<void> {
    // HACK: work around configuration not being synchronously available
    await new Promise(resolve => setTimeout(resolve, 100))
...

This is documented in sourcegraph.d.ts:

         * @todo This function throws an error if it is called synchronously in the extension's `activate`
         *       function. This will be fixed before beta. See the test "Configuration (integration) / is usable in
         *       synchronous activation functions".
         *

This issue tracks removing this limitation.