fix #4286: use Uint8Array.fromBase64 if present

[evanw]

With this change, esbuild's binary loader will now use the new Uint8Array.fromBase64 function unless it's unavailable in the configured target environment. If it's unavailable, esbuild's previous code for this will be used as a fallback.

Fixes #4286

[bakkot]

Might be worth also having the fallback implementation check for fromBase64 at runtime. That way users who need support for older browsers can still get the native (and likely faster) implementation for their consumers who are on newer browsers.