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.

vitest.config.ts

@@ -0,0 +1,11 @@
+import tsconfigPaths from "vite-tsconfig-paths";
+import { configDefaults, defineConfig, type UserConfig } from "vitest/config";
+
+const config = defineConfig({
+  test: {
+    ...configDefaults, // Extending Vitest's default options
+  },
+  plugins: [tsconfigPaths()],
+}) as UserConfig;
+
+export default config;