correctly report assignment to imported variable error in TypeScript

[strager]

The following is valid TypeScript if the imported x is a type-only declaration (e.g. an interface):

import {x} from './mod';
x = 42;
var x;

The following is invalid TypeScript even if the imported x is a runtime declaration:

import {x} from './mod';
x = 42;

We should fix quick-lint-js's variable analyzer to handle both of these cases properly.