refactor: Simplify Posthog.init() signature

In a recent version we updated the body of this function to always return a `Posthog` instance but we forgot to update the type, let's fix that

Originally written by https://github.com/thexeos but I couldn't get his PR to pass CI - secrets are not shared with contributors PRs

Closes #1616

Co-authored by thexeos

src/posthog-core.ts

@@ -368,7 +368,7 @@ export class PostHog {
         token: string,
         config?: OnlyValidKeys<Partial<PostHogConfig>, Partial<PostHogConfig>>,
         name?: string
-    ): PostHog | undefined {
+    ): PostHog {
         if (!name || name === PRIMARY_INSTANCE_NAME) {
             // This means we are initializing the primary instance (i.e. this)
             return this._init(token, config, name)