feat(stdlib): add vim.base64 module (#25843)

Add base64 encode() and decode() functions to a vim.base64 module.

runtime/lua/vim/_meta/base64.lua

@@ -0,0 +1,13 @@
+--- @meta
+
+--- Encode {str} using Base64.
+---
+--- @param str string String to encode
+--- @return string Encoded string
+function vim.base64.encode(str) end
+
+--- Decode a Base64 encoded string.
+---
+--- @param str string Base64 encoded string
+--- @return string Decoded string
+function vim.base64.decode(str) end