Extension store base class + rename main/renderer-extensions -> extensions (#1107)

Signed-off-by: Jari Kolehmainen <jari.kolehmainen@gmail.com>

src/extensions/core-api/event-bus.ts

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+export { appEventBus } from "../../common/event-bus"
+export type { AppEvent } from "../../common/event-bus"

src/extensions/core-api/registries.ts

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+export type { DynamicPageType, PageRegistry } from "../page-registry"
+export type { AppPreferenceRegistry } from "../app-preference-registry"

src/extensions/core-api/stores.ts

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+export { ExtensionStore } from "../extension-store"

src/extensions/core-api/utils.ts

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+export { Singleton } from "../../common/utils"