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/modelProviders/openai-ChatCompletion/index.ts

@@ -0,0 +1,54 @@
+import { type JSONSchema4 } from "json-schema";
+import { type ModelProvider } from "../types";
+import inputSchema from "./codegen/input.schema.json";
+import { type CompletionCreateParams } from "openai/resources/chat";
+
+const supportedModels = [
+  "gpt-4-0613",
+  "gpt-4-32k-0613",
+  "gpt-3.5-turbo-0613",
+  "gpt-3.5-turbo-16k-0613",
+] as const;
+
+type SupportedModel = (typeof supportedModels)[number];
+
+const modelProvider: ModelProvider<SupportedModel, CompletionCreateParams> = {
+  name: "OpenAI ChatCompletion",
+  models: {
+    "gpt-4-0613": {
+      name: "GPT-4",
+      learnMore: "https://openai.com/gpt-4",
+    },
+    "gpt-4-32k-0613": {
+      name: "GPT-4 32k",
+      learnMore: "https://openai.com/gpt-4",
+    },
+    "gpt-3.5-turbo-0613": {
+      name: "GPT-3.5 Turbo",
+      learnMore: "https://platform.openai.com/docs/guides/gpt/chat-completions-api",
+    },
+    "gpt-3.5-turbo-16k-0613": {
+      name: "GPT-3.5 Turbo 16k",
+      learnMore: "https://platform.openai.com/docs/guides/gpt/chat-completions-api",
+    },
+  },
+  getModel: (input) => {
+    if (supportedModels.includes(input.model as SupportedModel))
+      return input.model as SupportedModel;
+
+    const modelMaps: Record<string, SupportedModel> = {
+      "gpt-4": "gpt-4-0613",
+      "gpt-4-32k": "gpt-4-32k-0613",
+      "gpt-3.5-turbo": "gpt-3.5-turbo-0613",
+      "gpt-3.5-turbo-16k": "gpt-3.5-turbo-16k-0613",
+    };
+
+    if (input.model in modelMaps) return modelMaps[input.model] as SupportedModel;
+
+    return null;
+  },
+  inputSchema: inputSchema as JSONSchema4,
+  shouldStream: (input) => input.stream ?? false,
+};
+
+export default modelProvider;