refactor(sign): move legacy signs to extmarks

Problem:  The legacy signlist data structures and associated functions are
          redundant since the introduction of extmark signs.
Solution: Store signs defined through the legacy commands in a hashmap, placed
          signs in the extmark tree. Replace signlist associated functions.

Usage of the legacy sign commands should yield no change in behavior with the
exception of:
  - "orphaned signs" are now always removed when the line it is placed on is
    deleted. This used to depend on the value of 'signcolumn'.
  - It is no longer possible to place multiple signs with the same identifier
    in a single group on multiple lines. This will now move the sign instead.

Moreover, both signs placed through the legacy sign commands and through
|nvim_buf_set_extmark()|:
  - Will show up in both |sign-place| and |nvim_buf_get_extmarks()|.
  - Are displayed by increasing sign identifier, left to right.
    Extmark signs used to be ordered decreasingly as opposed to legacy signs.

src/nvim/highlight_defs.h

@@ -246,9 +246,3 @@ typedef struct {
 } ColorItem;
 #define COLOR_ITEM_INITIALIZER { .attr_id = -1, .link_id = -1, .version = -1, \
                                  .is_default = false, .link_global = false }
-
-/// highlight attributes with associated priorities
-typedef struct {
-  int hl_id;
-  int priority;
-} HlPriId;