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.

prisma/seed.ts

@@ -46,8 +46,10 @@ await prisma.promptVariant.createMany({
       label: "Prompt Variant 1",
       sortIndex: 0,
       model: "gpt-3.5-turbo-0613",
+      modelProvider: "openai/ChatCompletion",
+      constructFnVersion: 1,
       constructFn: dedent`
-        prompt = {
+        definePrompt("openai/ChatCompletion", {
           model: "gpt-3.5-turbo-0613",
           messages: [
             {
@@ -56,15 +58,17 @@ await prisma.promptVariant.createMany({
             }
           ],
           temperature: 0,
-        }`,
+        })`,
     },
     {
       experimentId: defaultId,
       label: "Prompt Variant 2",
       sortIndex: 1,
       model: "gpt-3.5-turbo-0613",
+      modelProvider: "openai/ChatCompletion",
+      constructFnVersion: 1,
       constructFn: dedent`
-        prompt = {
+        definePrompt("openai/ChatCompletion", {
           model: "gpt-3.5-turbo-0613",
           messages: [
             {
@@ -73,7 +77,7 @@ await prisma.promptVariant.createMany({
             }
           ],
           temperature: 0,
-        }`,
+        })`,
     },
   ],
 });