* 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
