Close #3600 - Implement pull diagnostics in VimL

Implement pull diagnostics in the VimL implementation so ALE is able
to track when servers are busy checking files. Only servers that
support this feature will return diagnostics these ways.

test/lsp/test_lsp_startup.vader

@@ -193,6 +193,10 @@ Before:
       \             'dynamicRegistration': v:false,
       \             'linkSupport': v:false,
       \           },
+      \           'diagnostic': {
+      \             'dynamicRegistration': v:true,
+      \             'relatedDocumentSupport': v:true,
+      \           },
       \           'publishDiagnostics': {
       \             'relatedInformation': v:true,
       \           },