fix(decorations): do not apply sign highlight id as range attr id

Problem:  ml_get error when scrolling after delete
Solution: mark topline to be validated in main_loop
          if it is larger than current buffers line
          count

reset_lnums() is called after e.g. TextChanged autocommands and it may
accidentally cause curwin->w_topline to become invalid, e.g. if the
autocommand has deleted some lines.

So verify that curwin->w_topline points to a valid line and if not, mark
the window to have w_topline recalculated in main_loop() in
update_topline() after reset_lnums() returns.

fixes: vim/vim#13568
fixes: vim/vim#13578

https://github.com/vim/vim/commit/c4ffeddfe5bd1824650e9b911ed9245bf56c69e3

The error doesn't happen in Nvim because Nvim triggers TextChanged after
calling update_topline().

Co-authored-by: Christian Brabandt <cb@256bit.org>

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

Problem:  Signcolumn width does not increase when ranged sign does not
          start at sentinel line.
Solution: Handle paired range of added sign when checking signcols.

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.