fix expert lsp stdio and register capability (#5113)

* pass --stdio flag to expert lsp server

expert requires --stdio to use stdio transport but the linter
definition launched it without the flag, so the server never
entered lsp mode

* handle client/registerCapability requests from lsp servers

some lsp servers like expert send a client/registerCapability
request after initialization to dynamically register capabilities,
if ale does not respond the server blocks and never processes
further messages like textDocument/didOpen

ale_linters/elixir/expert.vim

@@ -7,6 +7,6 @@ call ale#linter#Define('elixir', {
 \   'name': 'expert',
 \   'lsp': 'stdio',
 \   'executable': {b -> ale#Var(b, 'elixir_expert_executable')},
-\   'command': '%e',
+\   'command': '%e --stdio',
 \   'project_root': function('ale#handlers#elixir#FindMixUmbrellaRoot'),
 \})