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

docs/source/filters/base64_decode.md

@@ -0,0 +1,27 @@
+---
+title: base64_decode
+---
+
+{% since %}v10.24.0{% endsince %}
+
+Decodes a Base64-formatted string back to its original text.
+
+Input
+```liquid
+{{ "b25lIHR3byB0aHJlZQ==" | base64_decode }}
+```
+
+Output
+```text
+one two three
+```
+
+Input
+```liquid
+{{ "SGVsbG8sIFdvcmxkISBAIyQl" | base64_decode }}
+```
+
+Output
+```text
+Hello, World! @#$%
+```

docs/source/filters/base64_encode.md

@@ -0,0 +1,27 @@
+---
+title: base64_encode
+---
+
+{% since %}v10.24.0{% endsince %}
+
+Encodes a string into Base64 format.
+
+Input
+```liquid
+{{ "one two three" | base64_encode }}
+```
+
+Output
+```text
+b25lIHR3byB0aHJlZQ==
+```
+
+Input
+```liquid
+{{ "Hello, World! @#$%" | base64_encode }}
+```
+
+Output
+```text
+SGVsbG8sIFdvcmxkISBAIyQl
+```

docs/source/filters/overview.md

@@ -15,5 +15,6 @@ HTML/URI | escape, escape_once, url_encode, url_decode, strip_html, newline_to_b
 Array | slice, map, sort, sort_natural, uniq, where, where_exp, group_by, group_by_exp, find, find_exp, first, last, join, reverse, concat, compact, size, push, pop, shift, unshift
 Date | date, date_to_xmlschema, date_to_rfc822, date_to_string, date_to_long_string
 Misc | default, json, jsonify, inspect, raw, to_integer
+Base64 | base64_encode, base64_decode
 
 [shopify/liquid]: https://github.com/Shopify/liquid