Feature: add standalone else_if chaining for conditionals

[nholthaus]

Problem

A basic if / else construct can cover simple branching, but it does not provide a clean way to express multiple ordered conditions in one chain. Without else_if, scripts must nest conditionals manually, which makes branching logic harder to read and maintain.

Specification

Add support for standalone else_if chaining in script files.

Acceptance criteria:

• Scripts can use else_if as an additional branch within an if / else_if / else chain.
• else_if is treated as its own conditional construct and does not need to invoke or reuse the existing if implementation internally.
• At most one branch in the full chain executes.
• else_if remains compatible with the existing end_if terminator for the chain.
• Nested control-flow blocks remain parseable and unambiguous.
• Invalid or misplaced else_if usage fails with a clear error message.

Motivation

This would let scripts express multi-branch decisions more naturally without forcing extra nesting or duplicated control-flow structure.