feat(vite-plugin-markdown): support importing markdown files from outside root project

[MoustaphaDev]

Changes

This change adds support for importing markdown files from outside the root directory of a project, which is very common in monorepo setups

Testing

TODO - I'll need some assistance for this one

Docs

TODO - Not sure if it needs documentation but will do if guided /cc @withastro/maintainers-docs for feedback!