feat: Sort nodes sensibly

[sindrets]

Currently nodes don't always appear in conventional order in the file tree, and they are sometimes inserted in the wrong position when multiple nodes are added. This is a limitation of how insertion of new nodes is currently implemented. Further, symlinks are "sorted" as a separate group as opposed to how it is conventionally done: treating symlinks as the type of file/folder it points to. I.e a symlink pointing to a folder should be sorted alongside other folders.

Proposed solution: sort folders when new nodes are added. This is reasonable and does not have much notable impact on performance with my current implementation. Lua's table.sort is backed by an unstable quick sort algorithm, so I opted to instead implement a stable merge sort. Running some tests i measured a ~6.2ms runtime while sorting a folder with 5000 nodes.

Example behavior on current master while toggling dotfiles:

Peek.2021-04-21.23-13.mp4

.editorconfig and .luacheckrc are inserted at the wrong index and symlinks are sorted incorrectly.

Example behavior with this new implementation, while toggling dotfiles:

Peek.2021-04-21.23-11.mp4

All files retain their position in the tree after toggle and symlinks are sorted correctly.

[kyazdani42]

oh but by default it should be sorted i think. anyway this looks better :)

[sindrets]

It's sorted almost correctly after the first populate. Except for symlinks, because you group directories, links and files and then add them in that order.