Prep for more model providers

Adds a `modelProvider` field to `promptVariants`, currently just set to "openai/ChatCompletion" for all variants for now.

Adds a `modelProviders/` directory where we can define and store pluggable model providers. Currently just OpenAI. Not everything is pluggable yet -- notably the code to actually generate completions hasn't been migrated to this setup yet.

Does a lot of work to get the types working. Prompts are now defined with a function `definePrompt(modelProvider, config)` instead of `prompt = config`. Added a script to migrate old prompt definitions.

This is still partial work, but the diff is large enough that I want to get it in. I don't think anything is broken but I haven't tested thoroughly.

src/pages/_app.tsx

@@ -6,6 +6,7 @@ import Favicon from "~/components/Favicon";
 import "~/utils/analytics";
 import Head from "next/head";
 import { ChakraThemeProvider } from "~/theme/ChakraThemeProvider";
+import { SyncAppStore } from "~/state/sync";
 
 const MyApp: AppType<{ session: Session | null }> = ({
   Component,
@@ -20,6 +21,7 @@ const MyApp: AppType<{ session: Session | null }> = ({
         />
       </Head>
       <SessionProvider session={session}>
+        <SyncAppStore />
         <Favicon />
         <ChakraThemeProvider>
           <Component {...pageProps} />