We were previously unable to use simdutf for base64 decoding operations
other than "loose". Upstream has added support for the "strict" and
"stop-before-partial" operations, so let's make use of them!
These are going to be included in the ECMA-262 AOs once Temporal reaches
stage 4. There's no need to keep them in the Temporal namespace. Some
upcoming Temporal editorial changes will get awkward without this patch.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
