Fix resource-decl rule in grammar.

Allow for `resource x;` in the grammar (a resource with no constructor or methods).

Author: peterhuene

LANGUAGE.md

@@ -471,7 +471,7 @@ world-include-item  ::= id 'as' id
 world-ref           ::= package-path | id
 type-decl           ::= variant-decl | record-decl | flags-decl | enum-decl | type-alias
 item-type-decl      ::= resource-decl | type-decl
-resource-decl       ::= 'resource' id '{' resource-item* '}'
+resource-decl       ::= 'resource' id (';' | '{' resource-item* '}')
 resource-item       ::= constructor | method
 constructor         ::= 'constructor' param-list ';'
 method              ::= id ':' 'static'? func-type ';'