The __deleted__ string marker could collide with user data — if a user
set "__deleted__" as a Map key, it would be silently treated as a
deleted slot. Instead, mark entries as deleted by removing their key
property from the entries object. The existing entry_key_at function
already returns None for missing properties via the ? operator.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add Map, Set, WeakMap, and WeakSet constructors with prototype
methods to the JavaScript engine:

- Map: constructor (from iterable), set/get/has/delete/clear,
size property, forEach, keys/values/entries iterators.
SameValueZero key equality, insertion order preserved.

- Set: constructor (from iterable), add/has/delete/clear,
size property, forEach, keys/values/entries iterators.
SameValueZero value equality, insertion order preserved.

- WeakMap: constructor, set/get/has/delete. Keys must be objects.

- WeakSet: constructor, add/has/delete. Values must be objects.

Internal storage uses indexed entry objects on the GC heap with
deleted-slot markers for stable insertion-order iteration.

30 new integration tests covering all methods and edge cases
(NaN keys, object keys, chaining, uniqueness, constructor from
arrays, insertion order preservation).

Also removes Map/Set from test262 unsupported features list.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

The Dot (.) without the s flag should not match U+2028 (LINE SEPARATOR)
and U+2029 (PARAGRAPH SEPARATOR) per the ECMAScript spec, not just \n
and \r. Use the existing is_line_terminator helper for consistency.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>