feat(filters): Add base64_encode and base64_decode filters for Shopify compatibility (#828)

* feat(filters): add base64 encode and decode

* fix: use Object.defineProperty for cross-platform btoa/atob mocking

* docs(filters): update docs

* docs(filters): update version

src/build/base64-impl-browser.ts

@@ -0,0 +1,10 @@
+
+export function base64Encode (str: string): string {
+  return btoa(String.fromCharCode(...new TextEncoder().encode(str)))
+}
+
+export function base64Decode (str: string): string {
+  return new TextDecoder().decode(
+    Uint8Array.from(atob(str), c => c.charCodeAt(0))
+  )
+}