Problem:  Unable to identify legacy signs when fetching extmarks with
          `nvim_buf_get_extmarks()`.
Solution: Add "sign_name" to the extmark detail array.

Add some misc. changes as follow-up to #25724

Remove the monolithic Decoration struct. Before this change, each extmark
could either represent just a hl_id + priority value as a inline
decoration, or it would take a pointer to this monolitic 112 byte struct
which has to be allocated.

This change separates the decorations into two pieces: DecorSignHighlight
for signs, highlights and simple set-flag decorations (like spell,
ui-watched), and DecorVirtText for virtual text and lines.

The main separation here is whether they are expected to allocate more
memory. Currently this is not really true as sign text has to be an
allocated string, but the plan is to get rid of this eventually (it can
just be an array of two schar_T:s). Further refactors are expected to
improve the representation of each decoration kind individually. The
goal of this particular PR is to get things started by cutting the
Gordian knot which was the monolithic struct Decoration.

Now, each extmark can either contain chained indicies/pointers to
these kinds of objects, or it can fit a subset of DecorSignHighlight
inline.

The point of this change is not only to make decorations smaller in
memory. In fact, the main motivation is to later allow them to grow
_larger_, but on a dynamic, on demand fashion. As a simple example, it
would be possible to augment highlights to take a list of multiple
`hl_group`:s, which then would trivially map to a chain of multiple
DecorSignHighlight entries.

One small feature improvement included with this refactor itself, is
that the restriction that extmarks cannot be removed inside a decoration
provider has been lifted. These are instead safely lifetime extended
on a "to free" list until the current iteration of screen drawing is done.

NB: flags is a mess. but DecorLevel is useless, this slightly less so

Note: The crash happens in the second test case when using uninitialized
memory, and therefore doesn't happen with ASAN.

Problem:    Not easy to filter the output of maplist().
Solution:   Add mode_bits to the dictionary. (Ernie Rael, closes vim/vim#10356)

https://github.com/vim/vim/commit/d8f5f766219273a8579947cc80b92580b6988a4b

Co-authored-by: Ernie Rael <errael@raelity.com>

Problem:    It is not easy to restore saved mappings.
Solution:   Make mapset() accept a dict argument. (Ernie Rael, closes vim/vim#10295)

https://github.com/vim/vim/commit/51d04d16f21e19d6eded98f9530d84089102f925

Co-authored-by: Ernie Rael <errael@raelity.com>

Problem:    maparg() does not indicate the type of script where it was defined.
Solution:   Add "scriptversion".

https://github.com/vim/vim/commit/a9528b39a666dbaa026320f73bae4b1628a7fe51

Co-authored-by: Bram Moolenaar <Bram@vim.org>