feat(file-tree): integrate the file tree from pinelang/helix-tree-explorer

[usagi-flow]

This is the file tree feature which a PR was created for in the upstream project: https://github.com/helix-editor/helix/pull/5768

[usagi-flow]

The file tree explorer works pretty well, but I'm worried about the implementation not using a buffer as the underlying model for the file tree. This leads to bindings for movement that may have to be reimplemented, and it feels like the file tree isn't well integrated (e.g. attempting to move the cursor between windows won't move it to the file tree, which feels awkward).

It seems like the author of the upstream PR faced the same challenge: https://github.com/helix-editor/helix/pull/5768#issuecomment-1435638329