Add ELSEIF/ELSE branch-chain support to TEPL conditionals

[Copilot]

TEPL conditionals previously accepted only a single IF ... THEN ... END branch. This update adds native ELSEIF support (and ELSE THEN) so multi-branch logical flows parse and compile correctly.

• Grammar + tokens

  • Added ELSEIF and ELSE keywords.
  • Extended if_then_stmt to support:
    • IF ... THEN body
    • ELSEIF ... THEN body (0..n)
    • ELSE THEN body (0..1)
    • END

• AST model

  • Expanded If node to carry:
    • elif_branches: list[(condition, body)]
    • else_body: optional body

• Semantic validation + code generation

  • Added semantic checks for elif_branches and else_body.
  • Updated Python emission to generate if / elif / else blocks from the new AST shape.

• Focused coverage

  • Added tests that verify:
    • parser output includes ELSEIF/ELSE branches in AST
    • compiled output includes elif and else and executes the expected branch

if 1 == 2 then
    output "first"
elseif 2 == 2 then
    output "second"
else then
    output "third"
end

• Fixes Add ELSEIF #23

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