sorbet: stub output-chaining methods on MatcherDelegator

`RSpec::Matchers::MatcherDelegator` forwards unknown calls to its wrapped
`base_matcher` via `method_missing`. Sorbet cannot follow that delegation,
so aliased/negated wrappers of `output` (e.g. `not_to_output`) appear to
lack `to_stdout`, `to_stderr`, and their `_from_any_process` variants.

Declaring those four methods on `MatcherDelegator` in upstream.rbi — with
`T.self_type` as the return type to preserve chainability — eliminates the
"Method to_stderr does not exist on AliasedNegatedMatcher" errors (-129).