Note that we can currently only use simdutf for Base64 decoding if the
provided stopBeforePartial option is loose, which is the default. There
is an open issue for simdutf to implement strict and stop-before-partial
options. Until then, for those options, we implement a slow decoder that
is written exactly as the spec steps dictate.
See: https://github.com/simdutf/simdutf/issues/440