feat(statusline)!: allow %= in item groups, scope %< to item groups #40369

Problem: Separation markers (%=) are ignored within item groups. This
lead to a regression when the C implementation of the statusline was
replaced with a default expression. When the user configured a custom
ruler expression with a %= and used the overloaded item group syntax to
set the ruler width, the separation marker worked in the ruler, but not
when the ruler was incorporated into the statusline where the item group
syntax was interpreted in the usual way.

Solution: Analogously to top-level behaviour, expand separation markers
evenly within item groups until `minwid` is reached (if set).

ref https://github.com/neovim/neovim/pull/33036
fix https://github.com/neovim/neovim/issues/39984
ref https://github.com/neovim/neovim/issues/40247

Problem: The recursion offset into the static `stl_items` was not taken
into account when adjusting the item count after truncation.

Steps to reproduce: first prepare `stl_items`:
  set stl=%{%repeat('%#Error#',10)%}
then watch how the Error highlight leaks into the recursive call:
  set stl=%l%l%l%{%nvim_eval_statusline('test%l%<',{'maxwidth':3,'highlights':1}).highlights%}

ref https://github.com/neovim/neovim/issues/32259

* fix(statusline): consistent truncation at multicell character

Problem 1: truncation of item groups at multicell character didn't take
into account that minwid can be specified as a negative number.

Problem 2: after truncation at top-level from the right at multicell
character, the returned width was always `maxwidth`, even though the
actual width was reduced. In vim, this can be observed as a statusline
that is not fully drawn until the edge of the screen:
  vim --clean +"set ls=2 stl=%{%repeat('x',&columns-2)%}🙂x%<"

Problem 3: after truncation at top-level from the left at multicell
character, the resulting gap to reach `maxwidth` again was filled with
fillchars, but then the final NUL was not set correctly.
This can be seen in the following example, where the statuscolumn spills
into the editing area starting from line 10:
  nvim --clean +"set number stc=%<x🙂%{repeat('x',43)}%l" +"norm yy10p"

Solution: fix the small errors and, at top-level, consistently reduce
the size instead of compensating with fillchars. In the case of the
statusline and the winbar, the remaining place is filled with the
configured fillchars in `win_redr_custom`, after `build_stl_str_hl` has
returned. In all other cases (title, icon, statuscol, tabline, ruler),
there seems to be no point in adding additional spaces at the end.

* feat(statusline)!: scope %< to item groups

Problem:
Previously, item groups were only truncated at the beginning, which is
often not desired. In the example
  %.15(path: %f%)
the group's title/label is truncated away:
  <th/to/file.txt
Truncation markers (%<) in item groups were processed at the top-level
in the end, which can be confusing. Only the first %< is used for the
whole string, and it is used even if the containing item group is
hidden. Additionally, in the case of hidden item groups, the marker's
position was not adapted. For example,
  %(hidden%<%)%f
had the effect of truncating the path somewhere in the middle:
  /path/<file.txt

Solution:
Make truncation consistent with top-level behaviour, which has a better
default of truncating at the first `Normal` item, i.e.
  path: <file.txt
and allows for fine-grained control with truncation markers (%<). E.g.
  %.15(path: %f%<%)
now yields
  path: /path/to>
The original behaviour can be restored like so:
  %.15(%<path: %f%)

BREAKING CHANGE: %< is no longer processed at top-level
- the default truncation behaviour has changed: now at first item
- truncation markers inside item groups don't affect truncation outside
  of the item group anymore
- several truncation markers can now have an effect when separated with
  item groups, whereas previously only the first one globally had

ref https://github.com/neovim/neovim/issues/39984

runtime/lua/vim/_meta/options.gen.lua

@@ -6875,12 +6875,16 @@ vim.wo.stc = vim.wo.statuscolumn
 ---          this label.
 ---       Use `getmousepos()`.winid in the specified function to get the
 ---       corresponding `window-ID` of the clicked item.
---- \< -   Where to truncate line if too long.  Default is at the start.
+--- \< -   Where to truncate line if too long.  Default is at the first
+---       item.  Truncation markers within item groups apply to the
+---       truncation of that group until its maxwid is reached.
 ---       No width fields allowed.
 --- = -   Separation point between alignment sections.  Each section will
 ---       be separated by an equal number of spaces.  With one %= what
 ---       comes after it will be right-aligned.  With two %= there is a
 ---       middle part, with white space left and right of it.
+---       Alignment sections within item groups will be separated until
+---       minwid of the group is reached.
 ---       No width fields allowed.
 --- # -   Set highlight group.  The name must follow and then a # again.
 ---       Thus use %#HLname# for highlight group HLname.  The same