Support for vite ModuleRunner or other configLoader alternative

[neochief]

Since recently (vitejs/vite#5370), Vite got the ability to directly import ts files into config without transpiling by running vite like this vite --configLoader=runner. In react router CLI, this capability seem to be non-existent.

This feature is super useful in the monorepo context, because it lets you save some building time on transpiling the custom plugins (not to mention, the convenience).

I've created a PNPM monorepo to show case this use case: https://github.com/neochief/rr-ts-import

This repo has a pure-TS vite plugin that is connected to a regular Vite app and React Router app in exact the same way.

The only difference is that Vite app's build works fine, and React Router's doesn't:

> react-router-app@ build /rr-ts-import/apps/react-router-app
> react-router build

failed to load config from /rr-ts-import/apps/react-router-app/vite.config.ts
TypeError: Unknown file extension ".ts" for /rr-ts-import/packages/testPlugin/testPlugin.ts

Steps to install the test repo

# Install repo
clone https://github.com/neochief/rr-ts-import.git rr-ts-import
cd rr-ts-import
pnpm i

# Run the build in the Vite app (succeeds)
pnpm --filter=vite-app run build

# Run the build in the React Router app (fails)
pnpm --filter=react-router-app run build