fix(hsl): keep alpha when constructing from an object

[xianjianlf2]

Problem

chroma({ h, s, l, a }) dropped the alpha channel and returned an opaque color, while the equivalent rgb object form kept it. hsl2rgb already preserves alpha for array/positional input via its args.length > 3 branch, but it unpacked objects with the keyOrder 'hsl', which excludes the a key.

Fix

Unpack with 'hsla' (the same keyOrder css/hsl2css.js already uses) so object input reaches the existing alpha-preserving path. Array and positional callers are unaffected since unpack ignores keyOrder when given 3+ args or an array.

-    args = unpack(args, 'hsl');
+    args = unpack(args, 'hsla');

Testing

Added a regression test in test/alpha.test.js:

it('constructing hsla color from object keeps alpha (#219)', () => {
    const color = chroma({ h: 120, s: 0.8, l: 0.75, a: 0.5 });
    expect(color.alpha()).toBe(0.5);
    expect(color.rgba()).toEqual([140, 242, 140, 0.5]);
});

Closes #219

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