lsp: Enable formatting functionality

In #641 support was added to
handle textDocument/formatting requests from clients. However, the
functionality was disabled for fear that users would see a confusing
message:

#638 (comment)

In open-policy-agent/vscode-opa#148 the regal
formatter will be used if the user has enabled regal and opa fmt will be
used if not. This means that there is only ever one formatter available
for rego and the user will see no message.

Signed-off-by: Charlie Egan <charlie@styra.com>

internal/lsp/messages.go

@@ -160,7 +160,7 @@ type FormattingOptions struct {
 	InsertSpaces           bool `json:"insertSpaces"`
 	TrimTrailingWhitespace bool `json:"trimTrailingWhitespace"`
 	InsertFinalNewline     bool `json:"insertFinalNewline"`
-	TrimFinalNewlines      bool `json:"trimFinalNewline"`
+	TrimFinalNewlines      bool `json:"trimFinalNewlines"`
 }
 
 type FileOperationsServerCapabilities struct {

internal/lsp/server.go

@@ -565,7 +565,7 @@ func (l *LanguageServer) handleTextDocumentFormatting(
 	}
 
 	if warnings := validateFormattingOptions(params.Options); len(warnings) > 0 {
-		fmt.Fprintf(os.Stderr, "formatting params validation warnings: %v", warnings)
+		fmt.Fprintf(l.errorLog, "formatting params validation warnings: %v\n", warnings)
 	}
 
 	oldContent, ok := l.cache.GetFileContents(params.TextDocument.URI)
@@ -759,7 +759,7 @@ func (l *LanguageServer) handleInitialize(
 			ExecuteCommandProvider: ExecuteCommandOptions{
 				Commands: []string{"regal.fmt", "regal.fmt.v1"},
 			},
-			DocumentFormattingProvider: false,
+			DocumentFormattingProvider: true,
 		},
 	}