feat: accept a destructuring pattern as the for-of loop variable

[marevol]

Stacked on #57.

for-of (#39) and destructuring (#38) shipped separately and their combination did not parse, including the shape the two are most often used together in:

for (const [key, value] of pairs) { ... }   // Expected = but found of

Cause

isForOf() required an IDENT where the loop variable goes, so a pattern was never recognised as a for-of head at all and the of fell through to the last switch.

Fix

It now also accepts a bracket or a brace and scans to the bracket that closes it, the way isDestructuringAssignment does. That scan goes through lookahead() (#51), so it leaves the token stream alone and a regular expression in the source is still readable afterwards.

forOf() then parses the loop variable as a pattern and, per iteration, binds the element to a temporary and takes the pattern apart from there — so the element expression is evaluated once, as it is for a plain loop variable. The two existing emitters do the work: declareBindings for var, let and const, assignBindings for the form with no declaration, where the leaves are assignment targets rather than names to declare.

Covered: array and object patterns, renaming, rest, nesting, defaults, the pairs idiom, member expressions as targets, per-iteration let capture, and break/continue.

Not in this change

The same pattern in a for-in head. for-in yields keys, which are strings, so destructuring one is legal ES6 but of no practical use; for (var [a, b] in obj) stays a syntax error.

Verification

./gradlew build testOptimistic testPessimistic

suite	before	after
test	666, 0 fail	666, 0 fail
testOptimistic	1718, 0 fail	1718, 0 fail
testPessimistic	1718, 0 fail	1718, 0 fail