Fix get/set_embedder_data to create annex and offset by internal slot count

[coolreader18]

I didn't realize this when I added these functions in #1785, but they use the same slotspace as {Get,Set}AlignedPointerInEmbedderData, so we need to offset them by INTERNAL_SLOT_COUNT and create the annex before setting them.

[coolreader18]

Ping on this - it'd be nice to get this into a patch release.

[devsnek]

can you add a test?

[coolreader18]

In doing so I realized that the current set_aligned_pointer_in_embedder_data implementation is perhaps incorrect - the following code can cause a segfault:

static X: [u16; 2] = [1, 2];
unsafe { context.set_aligned_pointer_in_embedder_data(0, X.as_ptr() as _) };

because ANNEX_SLOT is 1 but INTERNAL_SLOT_COUNT is also 1. Should I fix that as well? Probably just by changing INTERNAL_SLOT_COUNT to 2?