feat(grammar): added support for map types, nested packages, external-ids, and fallible constructors

[mkatychev]

Created decl_head for the package_decl contents without semicolon.
This pattern, though not following the spec, makes it easier to reconcile with nested packages:

tree-sitter-wit/grammar.js

Lines 78 to 85 in c858306

	decl_head: $ =>
	seq(
	'package',
	repeat1($._uri_head),
	$.id,
	repeat($._uri_tail),
	optional($._version),
	),

Named interfaces and exports:

tree-sitter-wit/test/corpus/world.txt

Lines 303 to 314 in 00bea4a

	==================
	Named interface imports and exports
	==================
	world my-world {
	import primary: wasi:keyvalue/store;
	import secondary: wasi:keyvalue/store;
	export my-handler: wasi:http/handler;
	import cache: store;
	}
	---