Don't add sign extmarks to state->active. Instead when drawing signs,
perform a full line scan for sign marks. This allows decor_redraw_line
to be moved back inside the `!number_only` block in screen.c, which
prevents decor scans when redrawing the number column when
'relativenumber' is set.
Fixes: #17638
Add the following options to extmarks:
- sign_text
- sign_hl_group
- number_hl_group
- line_hl_group
- cursorline_hl_group
Note: ranges are unsupported and decorations are only applied to
start_row
Since the providers are ordered by ns_id, inserting a new provider may
require shifting existing providers around to maintain this ordering.
When this happens, we need to allocate a new element at the end of the
vector and then shift the larger elements to the right. Rather than
iterating (incorrectly) with a loop and copying each item, use memmove
to copy the entire block.
