Before this commit, the LSP client would throw errors when messages
without severity would be sent by the server. We make severity default
to `Error` as a kludge before proper heuristics to discover the severity
of a message are found.
According to the LSP specification, showMessage is what is displayed and logMessage is what is stored.
Since these are different things, I devide the callback into those that match.
fix: #11826
Some lanuguage servers return complementary candidates whose prefixes do not match are also returned.
So we exclude completion candidates whose prefix does not match.
ex) Microsoft python-language-server, rust-analyzer
- In the process, refactored focusable_preview to a util function.
- Add text for locations_to_items of the current line.
- Improve location callback to handle multiple return values by using
set_qflist.
- Remove update_tagstack and leave note for future travelers.
