types: export Sink, Source and HeaderArgument

[PE39806]

Sink, Source and HeaderArgument are all reachable from the public API but are declared locally and left out of the export list, so consumers cannot use them.

• Sink is the return type of pack.entry()
• Source is the stream passed to the entry event, and the item type of Extract's AsyncIterable
• HeaderArgument is the parameter type of pack.entry()

Without them, anything that wraps, mocks or delegates to entry(), or declares a standalone entry handler, has to fall back on ReturnType<Pack['entry']> and Parameters<Pack['entry']>[0].

This also fills in Sink, which was declared as an empty interface. pack.js sets header and written on it (without an underscore prefix, unlike the internal fields). entry() normalises size, type, mode, uid, gid and mtime before constructing the sink, so these will always be present. The remaining header fields stay optional.