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.

.github/workflows/auto_label_prs.yaml

@@ -63,7 +63,6 @@ jobs:
             toolchain:
               - 'common/**'
               - 'core/**'
-              - 'language_server/**'
               - 'testing/**'
               - 'toolchain/**'