[base64] Add FromBase64

This CL adds `Uint8Array.fromBase64()`.

Bug: 42204568
Change-Id: Ib68683a2d5ead9720077197c0f895787214b183e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/6026354
Commit-Queue: Rezvan Mahdavi Hezaveh <rezvan@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#98239}

Author: rmahdav

test/staging/Uint8Array/fromBase64/invalid-options.js

@@ -0,0 +1,55 @@
+// Copyright (C) 2025 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-uint8array.frombase64
+description: >
+  Uint8Array.fromBase64 throws a TypeError when alphabet and
+  lastChunkHandling are invalid string values.
+includes: [compareArray.js]
+features: [uint8array-base64, TypedArray]
+---*/
+
+let string = 'SGVsbG8gV29ybGQ=';
+assert.compareArray(
+    Uint8Array.fromBase64(string),
+    [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100])
+
+// invalid alphabet -----
+
+// shorter length
+assert.throws(TypeError, function() {
+  Uint8Array.fromBase64(string, {alphabet: 'base'});
+});
+// same length but invalid value
+assert.throws(TypeError, function() {
+  Uint8Array.fromBase64(string, {alphabet: 'base65'});
+});
+// longer length
+assert.throws(TypeError, function() {
+  Uint8Array.fromBase64(string, {alphabet: 'base64urlurl'});
+});
+// invalid two-byte value
+assert.throws(TypeError, function() {
+    Uint8Array.fromBase64(string, {alphabet: '☉‿⊙'});
+  });
+
+// invalid lastChunkHandling -----
+
+// shorter length
+assert.throws(TypeError, function() {
+  Uint8Array.fromBase64(string, {lastChunkHandling: 'stric'});
+});
+// same length but invalid value
+assert.throws(TypeError, function() {
+  Uint8Array.fromBase64(string, {lastChunkHandling: 'looss'});
+});
+// longer length
+assert.throws(TypeError, function() {
+  Uint8Array.fromBase64(
+      string, {lastChunkHandling: 'stop-before-partial-partial'});
+});
+// invalid two-byte value
+assert.throws(TypeError, function() {
+    Uint8Array.fromBase64(string, {lastChunkHandling: '☉‿⊙'});
+  });