Conform dynamic imports `import('a')` to the new ESTree spec

[bradzacher]

For this code example:

const x = import('module');

We currently emit the following AST (excluding the variable decl):

{
  "type": "CallExpression",
  "callee": {
    "type": "Import",
  },
  "arguments": [
    {
      "type": "Literal",
      "raw": "'a'",
      "value": "a",
    }
  ],
  "optional": false,
}

When we should be emitting this (as per the ESTree spec)

{
  "type": "ImportExpression",
  "source": {
    "type": "Literal",
    "value": "a",
    "raw": "'a'"
  }
}