This PR attempts to provide an option to allow importing TS/TSX from outside of the current Next.js project root directory. Although this goes against the design decision that no source code should be imported from outside of root and [might bring tons of issues](https://github.com/vercel/next.js/issues/19928#issuecomment-741596557), it will still be helpful in some monorepo use cases.
This PR assumes that the external files are following the same language syntax rules and under the same tooling versions as the source code inside your project root. And it's also not allowed to enable the `baseUrl` feature in the external directory (as the project should only have 1 import base URL).
X-ref: #9474, #15569, #19928, #20374.
