LibJS: Add String.prototype.pad{Start,End}()

Author: https://github.com/linusg
Commit: 31505dde7e
Pull-request: https://github.com/SerenityOS/serenity/pull/1731
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Tests/String.prototype.padEnd.js

@@ -0,0 +1,22 @@
+try {
+    assert(String.prototype.padEnd.length === 1);
+
+    var s = "foo";
+    assert(s.padEnd(-1) === "foo");
+    assert(s.padEnd(0) === "foo");
+    assert(s.padEnd(3) === "foo");
+    assert(s.padEnd(5) === "foo  ");
+    assert(s.padEnd(10) === "foo       ");
+    assert(s.padEnd("5") === "foo  ");
+    assert(s.padEnd([[["5"]]]) === "foo  ");
+    assert(s.padEnd(2, "+") === "foo");
+    assert(s.padEnd(5, "+") === "foo++");
+    assert(s.padEnd(5, 1) === "foo11");
+    assert(s.padEnd(10, null) === "foonullnul");
+    assert(s.padEnd(10, "bar") === "foobarbarb");
+    assert(s.padEnd(10, "123456789") === "foo1234567");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}

Libraries/LibJS/Tests/String.prototype.padStart.js

@@ -0,0 +1,22 @@
+try {
+    assert(String.prototype.padStart.length === 1);
+
+    var s = "foo";
+    assert(s.padStart(-1) === "foo");
+    assert(s.padStart(0) === "foo");
+    assert(s.padStart(3) === "foo");
+    assert(s.padStart(5) === "  foo");
+    assert(s.padStart(10) === "       foo");
+    assert(s.padStart("5") === "  foo");
+    assert(s.padStart([[["5"]]]) === "  foo");
+    assert(s.padStart(2, "+") === "foo");
+    assert(s.padStart(5, "+") === "++foo");
+    assert(s.padStart(5, 1) === "11foo");
+    assert(s.padStart(10, null) === "nullnulfoo");
+    assert(s.padStart(10, "bar") === "barbarbfoo");
+    assert(s.padStart(10, "123456789") === "1234567foo");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}