refactor: extract shared code for linting if-else chains (#821)

* refactor: extract shared code for linting if-else chains

The rules "early-return", "indent-error-flow" and
"superfluous-else" have a similar structure. This
moves the common logic for classifying if-else chains
to a common package.

A few side benefits:
- "early-return" now handles os.Exit/log.Panicf/etc
- "superfluous-else" now handles (builtin) panic
- "superfluous-else" and "indent-error-flow" now handle if/else
  chains with 2+ "if" branches

* internal/ifelse: style fixes, renames, spelling

internal/ifelse/doc.go

@@ -0,0 +1,6 @@
+// Package ifelse provides helpers for analysing the control flow in if-else chains,
+// presently used by the following rules:
+// - early-return
+// - indent-error-flow
+// - superfluous-else
+package ifelse