The `FileLocation` and `PathOrId` types can borrow paths rather than
requiring them to be owned. This takes a refactor of the preview
functions and preview internals within `Picker`. With this change we
avoid an unnecessary `PathBuf` clone per render for any picker with a
file preview function (i.e. most pickers).
This refactor is not fully complete. The `PathOrId` is _sometimes_ an
owned `PathBuf`. This is for pragmatic reasons rather than technical
ones. We need a further refactor to introduce more core types like
`Location` in order to eliminate the Cow and only use `&Path`s within
`PathOrId`. This is left for future work as it will be a larger refactor
almost entirely fitting into the LSP commands module and helix-core -
i.e. mostly unrelated to refactoring the `Picker` code itself.
Co-authored-by: Pascal Kuthe <pascalkuthe@pm.me>
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