Diagnose statement in namespace

[HerrCai0907]

Bug description

statement in namespace does not have any effect. It should be diagnosed.

Steps to reproduce

let a = 0;

namespace NS {
  a = 100; // here should be diagnosed
  export function foo(): void {
    trace("a", 1, a);
  }
}

export function _start(): void {
  trace("a", 1, a);
  NS.foo();
}

compile and run this code will get

a 0
a 0

AssemblyScript version

0.27.x

[HerrCai0907]

from #2763

[HerrCai0907]

in typescript, a should be 100 in both position

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports._start = void 0;
var a = 0;
var NS;
(function (NS) {
    a = 100;
    function foo() {
        console.log("a", 1, a);
    }
    NS.foo = foo;
})(NS || (NS = {}));
function _start() {
    console.log("a", 1, a);
    NS.foo();
}
exports._start = _start;
_start();

[JesseCodeBones]

according to typescript grammar, namespace can have top level statement.

namespaceDeclaration
    : Namespace namespaceName '{' statementList? '}'
    ;
statementList
    : statement+
    ;

so parse and compile top level statement should be reasonable in namespace