Handle Base64 in uris

[CedricGuillemet]

handle base64 data uris generically instead of specific case like BabylonJS/BabylonNative#1328

[bkaradzic-microsoft]

Being addressed by #212 (open) — Support data URLs in the fetch polyfill, which resolves both percent-encoded and Base64 data: URLs generically (WHATWG forgiving-base64 validation, default/explicit media types, fragment stripping, case-insensitive base64, missing padding, invalid-alphabet rejection), with cases ported from WPT fetch/data-urls and Chromium's DataURL tests — rather than the one-off decode this issue was filed against (BabylonJS/BabylonNative#1328).

Note on scope: #212 implements data: inside Polyfills/Fetch/Source/Fetch.cpp, so it covers the fetch path only. XMLHttpRequest and other UrlRequest consumers (image/video src, texture loaders) still won't resolve data:. The fully generic answer is the same shape as the blob: work in #207/#213 — a data: scheme resolver registered through UrlLib::UrlRequest::RegisterSchemeResolver, so every consumer gets it uniformly.

Leaving this open until that generic path exists; #212 is the first (and largest) piece.