Add `BugChecker.SwitchExpressionTreeMatcher` to support checking of `switch` expressions

Right now, Error Prone checkers cannot perform checks on `switch` expressions as there is not yet a `BugChecker.SwitchExpressionTreeMatcher` interface to override.  See also discussion at https://github.com/google/error-prone/issues/3803#issuecomment-1736477471.