fix(templates): generates import map with given project name and misc housekeeping updates

[jessrynkar]

What?

1. Adds logic to automatically update the importMap.js file with the project name provided by the user.
2. Adds an updated version of the README.md file that we had when this template existed outside of the monorepo (here) to provide clear instructions of required steps.

Why?

1. The plugin template when installed via npx create-payload-app asks the user for a project name, however the exports from importMap.js do not get updated to the provided name. This throws errors when running the project and prevents it from building.

2. The /dev folder requires the .env.example to be copied and renamed to .env - the project will not run until this is done. The template lacks instructions that this is a required step.

How?

1. Updates packages/create-payload-app/src/lib/configure-plugin-project.ts to read the importMap.js file and replace the placeholder plugin name with the name provided by the users. Adds a test to packages/create-payload-app/src/lib/create-project.spec.ts to verify that this file gets updated correctly.
2. Adds instructions on using this template to the README.md file, ensuring key steps (like adding the .env file) are clearly stated.

Additional housekeeping updates:

• Removed Jest and replaced it with Vitest for testing
• Updated the base test approach to use Vitest instead of Jest
• Removed NextRESTClient in favor of directly creating Request objects
• Abstracted getCustomEndpointHandler function
• Added ensureIndexes: true to the mongooseAdapter configuration
• Removed the custom server from the dev folder
• Updated the pnpm dev script to "dev": "next dev dev --turbo"
• Removed admin.autoLogin

Fixes #12198