Problem:  No test for mode() when executing Ex commands
Solution: Add some test cases and simplify several other test cases.
          Also add a few more test cases for ModeChanged.

closes: vim/vim#13588

https://github.com/vim/vim/commit/fcaeb3d42b228e73c669b2fce78f1d3fe112769f

feat(decoration): allow conceal_char to be a composing char

decor->text.str pointer must go. This removes it for conceal char,
in preparation for a larger PR which will also handle the sign case.

By actually allowing composing chars for a conceal chars, this
becomes a feature and not just a refactor, as a bonus.

Problem:  Cannot detect overstrike mode in Cmdline mode
Solution: Make mode() return "cr" for overstrike

closes: vim/vim#13569

https://github.com/vim/vim/commit/d1c3ef1f47c87d1da056c56564e1985fe6f2931d

`vim.wait()` cannot be called in a fast callback since the main loop
cannot be run in that context as it is not reentrant

Fixes #26122

runtime(doc): Fix whitespace and formatting of some help files (vim/vim#13549)

https://github.com/vim/vim/commit/596a9f29c83af85ace1a2702c88591851ad14df8

N/A patch:
vim-patch:aabca259fa48

Co-authored-by: h_east <h.east.727@gmail.com>

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.