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.

1 files changed, 1 insertions, 0 deletions

diff --git a/test/functional/ui/statuscolumn_spec.lua b/test/functional/ui/statuscolumn_spec.lua
index 9f00c7a5d6..61f6419f8c 100644
--- a/test/functional/ui/statuscolumn_spec.lua
+++ b/test/functional/ui/statuscolumn_spec.lua
@@ -377,6 +377,7 @@ describe('statuscolumn', function()
                                                            |
     ]])
     command('set breakindent')
+    command('sign unplace 2')
     feed('J2gjg0')
     screen:expect([[
       {2: }{4: 0│}{1:>>}{2:                }{4: }{5:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa}|