Adding support for LSP window/showMessage method (#2652)

* Added base handling for window/showMessage
* Ignoring severity log
* Code formatting
* Added user configurable severity
* Preferring ale#util#ShowMessage over echo'ing directly
* Using format similar to ale_echo_msg_format for consistency
* Updating docs
* Added LSP log config string; improved tests
* Use warning as fallback for incorrect user config

autoload/ale/lsp_linter.vim

@@ -130,6 +130,12 @@ function! ale#lsp_linter#HandleLSPResponse(conn_id, response) abort
         call s:HandleLSPErrorMessage(l:linter_name, a:response)
     elseif l:method is# 'textDocument/publishDiagnostics'
         call s:HandleLSPDiagnostics(a:conn_id, a:response)
+    elseif l:method is# 'window/showMessage'
+        call ale#lsp_window#HandleShowMessage(
+        \   s:lsp_linter_map[a:conn_id],
+        \   g:ale_lsp_show_message_format,
+        \   a:response.params
+        \)
     elseif get(a:response, 'type', '') is# 'event'
     \&& get(a:response, 'event', '') is# 'semanticDiag'
         call s:HandleTSServerDiagnostics(a:response, 'semantic')