* lsp: Remove duplicate `diagnostics` fallback in diagnostic.display
* lsp: Expose all diagnostics
Before the changes in #12655 it was possible to retrieve all diagnostics
via `vim.lsp.util.diagnostics_by_buf`.
This adds a `diagnostic.get_all()` to enable users to retrieve all
diagnostics.
Use cases for that could include loading all diagnostics into the
quickfix list, or to build an enhanced goto_next that can move across
buffers.