Tooling should be able to assume $PRJ_ROOT if it is not set

[gytis-ivaskevicius]

The spec does not define how $PRJ_ROOT should be assumed if it is not set.

Recommended implementation:
Default based on .editorconfig

When opening a file, EditorConfig plugins look for a file named .editorconfig in the directory of the opened file and in every parent directory. A search for .editorconfig files will stop if the root filepath is reached or an EditorConfig file with root=true is found.

If .editorconfig does not exist - default based on git/svn

One may argue that it should also default to build tool configs like package.json/Cargo.toml but I should note that just because such a file exists it does not necessarily mean that it is a project root since it may be a submodule. The same goes for most build tools