Rule Request: prefer_else_if

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

When an else branch has only one child statement which is either an if or if-else statement, this could be refactored to an else-if statement.

2. Provide several examples of what would and wouldn't trigger violations.

Violating Examples:

if foo {
  bar()
} else {
  if baz {
    qux()
  }
}

if foo {
  bar()
} else {
  if baz {
    qux()
  } else {
    quux()
  }
}

Non-Violating Examples:

if foo {
  bar()
} else {
  if baz {
    qux()
  }
  somethingElse()
}

if foo {
  bar()
} else if baz {
  qux()
}

if foo {
  bar()
} else if {
  qux()
} else {
  quux()
}

3. Should the rule be configurable, if so what parameters should be configurable?

This rule doesn’t need to be configurable.

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a rule as opt-in.

I think this should be an opt-in rule, because it is more like a stylistic rule to recommends to avoid unnecessary nesting.

Read README.md's opt-in-rules guidance, then compare the supplied violating and non-violating Swift examples with existing SwiftLint rule implementations and tests. The finished rule should flag an else branch containing only one if or if-else statement, while leaving multi-statement branches and existing else-if chains unchanged.