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/types.ts

@@ -0,0 +1,14 @@
+import { type JSONSchema4 } from "json-schema";
+
+export type ModelProviderModel = {
+  name: string;
+  learnMore: string;
+};
+
+export type ModelProvider<SupportedModels extends string, InputSchema> = {
+  name: string;
+  models: Record<SupportedModels, ModelProviderModel>;
+  getModel: (input: InputSchema) => SupportedModels | null;
+  shouldStream: (input: InputSchema) => boolean;
+  inputSchema: JSONSchema4;
+};