fix(js): build else branches into the CFG (#206)

[DaisukeYoda]

Fixes #206.

The parser kept the alternative of an if statement as a generic else_clause node. The CFG builder did not look inside it, so the statements in an else were never built into the CFG. An else if added no decision point, and a return in an else did not end its branch.

buildIfStatement now stores the statement inside the else_clause as Alternate: a block, or the next if of an else if chain. Nesting depth no longer needs to unwrap the clause, so isElseIf is removed.

Effects:

• Every else if adds one to cyclomatic complexity. On imbue-ai/latchkey total complexity rises from 2448 to 2463, and on sindresorhus/got from 9924 to 9988. Health scores do not change on either repo.
• Code after an if/else whose branches all return is reported as unreachable_branch.
• The else subtree loses one wrapper node, so clone similarity changes slightly (got: 2146 to 2139 clone pairs).