Only call workspace/configuration when available

Not all clients implement the client capability: `configuration`, which
was added in version 3.6.0 of the Language Server Protocol. The LSP
Specification also states:

> A missing property should be interpreted as an absence of the capability.

Above claims are possible to verify by reading the mentioned spec. at:
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_configuration

Hence this change modifies behaviour to only call the method on clients
explicitly announcing to support it.

This commit makes the lua-language-server work with vim-ale.

Thanks to Tom Lau for assistance in getting the test-setup updated.

Author: cos

script/lclient.lua

@@ -83,6 +83,9 @@ local defaultClientOptions = {
                 },
             },
         },
+        workspace = {
+            configuration = true,
+        },
     },
 }
 

script/provider/provider.lua

@@ -41,7 +41,10 @@ function m.updateConfig(uri)
     end
 
     for _, folder in ipairs(scope.folders) do
-        local clientConfig = cfgLoader.loadClientConfig(folder.uri)
+        local clientConfig = nil
+        if client.getAbility('workspace.configuration') then
+            clientConfig = cfgLoader.loadClientConfig(folder.uri)
+        end
         if clientConfig then
             log.info('Load config from client', folder.uri)
             log.info(inspect(clientConfig))
@@ -57,10 +60,12 @@ function m.updateConfig(uri)
         config.update(folder, clientConfig, rc)
     end
 
-    local global = cfgLoader.loadClientConfig()
-    log.info('Load config from client', 'fallback')
-    log.info(inspect(global))
-    config.update(scope.fallback, global)
+    if client.getAbility('workspace.configuration') then
+        local global = cfgLoader.loadClientConfig()
+        log.info('Load config from client', 'fallback')
+        log.info(inspect(global))
+        config.update(scope.fallback, global)
+    end
 end
 
 function m.register(method)