Move the language server into toolchain's busybox. (#4469)

Removes the separate language server binary; I'm not sure we need to
provide it. Instead, `carbon language-server` is added as a subcommand.

Moves //language_server to //toolchain/language_server. Splits into a
trivial language_server.h, and a substantive server.h. I wasn't sure
about a better name, but wanted the split similar to check/check.h,
lex/lex.h, etc. At the same time, the class is probably going to be a
little big so not a good fit to through into just a cpp file.

This fixes some style issues with the language server class, but
generally I'm trying to not address things here in order to keep it
simpler.

toolchain/language_server/language_server.cpp

@@ -0,0 +1,17 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/language_server/language_server.h"
+
+#include "toolchain/language_server/server.h"
+
+namespace Carbon::LanguageServer {
+
+auto Run(std::FILE* input_stream, llvm::raw_ostream& output_stream)
+    -> ErrorOr<Success> {
+  Server server(input_stream, output_stream);
+  return server.Run();
+}
+
+}  // namespace Carbon::LanguageServer