refactor: move vim.lsp.diagnostic to vim.diagnostic

This generalizes diagnostic handling outside of just the scope of LSP.
LSP clients are now a specific case of a diagnostic producer, but the
diagnostic subsystem is decoupled from the LSP subsystem (or will be,
eventually).

More discussion at [1].

[1]: #15585

runtime/doc/deprecated.txt

@@ -55,6 +55,50 @@ Functions ~
 			without stopping the job. Use chanclose(id) to close
 			any socket.
 
+LSP Diagnostics ~
+
+For each of the functions below, use the corresponding function in
+|vim.diagnostic| instead (unless otherwise noted). For example, use
+|vim.diagnostic.get()| instead of |vim.lsp.diagnostic.get()|.
+
+*vim.lsp.diagnostic.clear()*		Use |vim.diagnostic.hide()| instead.
+*vim.lsp.diagnostic.disable()*
+*vim.lsp.diagnostic.display()*		Use |vim.diagnostic.show()| instead.
+*vim.lsp.diagnostic.enable()*
+*vim.lsp.diagnostic.get()*
+*vim.lsp.diagnostic.get_all()*		Use |vim.diagnostic.get()| instead.
+*vim.lsp.diagnostic.get_count()*	Use |vim.diagnostic.get()| instead.
+*vim.lsp.diagnostic.get_line_diagnostics()*
+					Use |vim.diagnostic.get()| instead.
+*vim.lsp.diagnostic.get_next()*
+*vim.lsp.diagnostic.get_next_pos()*
+*vim.lsp.diagnostic.get_prev()*
+*vim.lsp.diagnostic.get_prev_pos()*
+*vim.lsp.diagnostic.get_virtual_text_chunks_for_line()*
+			Use |vim.diagnostic.get_virt_text_chunks()| instead.
+*vim.lsp.diagnostic.goto_next()*
+*vim.lsp.diagnostic.goto_prev()*
+*vim.lsp.diagnostic.redraw()*		Use |vim.diagnostic.show()| instead.
+*vim.lsp.diagnostic.reset()*
+*vim.lsp.diagnostic.save()*		Use |vim.diagnostic.set()| instead.
+*vim.lsp.diagnostic.set_loclist()*	Use |vim.diagnostic.setloclist()| instead.
+*vim.lsp.diagnostic.set_qflist()*	Use |vim.diagnostic.setqflist()| instead.
+*vim.lsp.diagnostic.show_line_diagnostics()*
+*vim.lsp.diagnostic.show_position_diagnostics()*
+
+The following are deprecated without replacement. These functions are moved
+internally and are no longer exposed as part of the API. Instead, use
+|vim.diagnostic.config()| and |vim.diagnostic.show()|.
+
+*vim.lsp.diagnostic.set_signs()*
+*vim.lsp.diagnostic.set_underline()*
+*vim.lsp.diagnostic.set_virtual_text()*
+
+LSP Utility Functions ~
+
+*vim.lsp.util.set_qflist()*		Use |setqflist()| instead.
+*vim.lsp.util.set_loclist()*		Use |setloclist()| instead.
+
 Lua ~
 *vim.register_keystroke_callback()* Use |vim.on_key()| instead.