fix background-* spelling

[fshowalter]

Closes #214

[asamuzaK]

Thanks for the catch.
Please add regression test at the end of test/CSSDeclaration.test.js:

describe("regression test for https://github.com/jsdom/cssstyle/issues/214", () => {
  it("should return value for each property", () => {
    const style = new CSSStyleDeclaration();
    const key = "background-color";
    const camel = "backgroundColor";
    const value = "var(--foo)";
    style[key] = value;
    assert.strictEqual(style[key], value);
    style[key] = null;
    style[camel] = value;
    assert.strictEqual(style[camel], value);
  });
});

[fshowalter]

Done. As an alternative, have you considered adding a cspell lint stage?

[RSS1102]

Done. As an alternative, have you considered adding a cspell lint stage?

use typos ？

[liweijie0812]

ci spelling check #216