Distinguish between "common" and "scripting" modes

[MaxDesiatov]

In "scripting" mode AST should preserve the order of top-level declarations, which probably means using different AST types and name resolution rules. On the top level the AST would probably be just an array of nodes, each can be either a declaration or an expression:

protocol ScriptNode {
}

extension Expr: ScriptNode {}
extension Decl: ScriptNode {}

typealias ScriptFile = [ScriptNode]