Check for project-root before adding overload

[Lunderberg]

Prior to emacs 28, the project.el package did not provide project-root. By providing an overload for project-root, any compatibility checks of (fboundp 'project-root) done by other packages will return true, even for versions that do not have project-root.

Fixes https://github.com/bbatsov/projectile/issues/1868.

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