cmd: add chain subcommand

Chain subcommand includes the commands: getblock, getbestblock, and getblockhash.

This commit removes conflicting neutrino cli commands.

cmd/lncli/chainrpc_default.go

@@ -0,0 +1,11 @@
+//go:build !chainrpc
+// +build !chainrpc
+
+package main
+
+import "github.com/urfave/cli"
+
+// chainCommands will return nil for non-chainrpc builds.
+func chainCommands() []cli.Command {
+	return nil
+}