[FIX] Switch to maintained fork of "css" package

This had been first attempted in 0abe66a but later reverted by 3b6c459
due to adobe/css-tools#77, reapplying since
the issue is now fixed.

---

This solves the issue of having outdated and potential insecure
transitive dependencies.

There are no known behavior changes, so this is considered a
non-breaking change / fix.

See: reworkcss/css#163
See: SamVerschueren/decode-uri-component#6

lib/index.js

@@ -1,7 +1,7 @@
 "use strict";
 
 const path = require("path");
-const css = require("css");
+const css = require("@adobe/css-tools");;
 
 
 const diff = require("./diff");

package.json

@@ -83,7 +83,7 @@
 	},
 	"dependencies": {
 		"clone": "^2.1.2",
-		"css": "^3.0.0",
+		"@adobe/css-tools": "^4.0.2",
 		"mime": "^1.6.0"
 	},
 	"devDependencies": {

test/test-diff.js

@@ -2,7 +2,7 @@
 "use strict";
 
 const assert = require("assert");
-const css = require("css");
+const css = require("@adobe/css-tools");;
 const fs = require("fs");
 const path = require("path");