refactor(ruler)!: replace C implementation with 'rulerformat' expression

Problem:
The default 'ruler' is implemented in C instead of the 'statusline' DSL.

Solution:
Replace the C implementation with a default 'rulerformat' expression.
This is a continuation of #1248 and #33036.

Advantages:
- configuration is more discoverable, the default being a useful example
- users and plugins can augment the default
- code reuse and less C code to maintain
- ui2: due to the use of an item group with `minwid`, it can expand
  instead of truncating when the content gets too long, which is
  particularly useful for locales with long translations of Top/Bot/All

Implementation details:
As is the case for 'statusline', when trying to set 'rulerformat' to an
empty string, the default expression is restored instead, mimicking how
previously the default C implementation would have been activated.
Just like before, `:set rulerformat=` and `:set rulerformat&` have the
same effect, and the ruler is disabled with `:set noruler`.

The default expression uses an item group with `%=`, unlike the fallback
in the previous default statusline `%-14.(%l,%c%V%) %P`, because the
total width and how it is configured is immediately clear without
documentation, it is a more useful pattern in general that works when
both sides have flexible width, and it also works for vim, which is
useful for configuration sharing/reuse.

A truncation marker `%<` is added at the end to mimic how at small
screen widths, the scroll percentage would disappear first, so that the
cursor position can remain fully visible.

BREAKING CHANGES:
- `&rulerformat` can no longer be set to an empty string
- ui2: the default ruler is no longer of fixed width, but can expand
- at very small screen widths (< 36 columns)
  - ui2: it will no longer try to shrink white-space before truncating
  - it truncates gradually from the right, whereas previously, the
    scroll percentage would disappear all at once
- l10n can no longer add a space after the comma between line and column
  (this was only done for one language: Ukrainian)

test/functional/testnvim.lua

@@ -27,7 +27,8 @@ M.nvim_set = (
   'set shortmess+=IS background=light noswapfile noautoindent startofline'
   .. ' laststatus=1 undodir=. directory=. viewdir=. backupdir=.'
   .. " belloff= wildoptions-=pum joinspaces noshowcmd noruler nomore redrawdebug=invalid shada=!,'100,<50,s10,h"
-  .. [[ statusline=%<%f\ %{%nvim_eval_statusline('%h%w%m%r',\ {'maxwidth':\ 30}).width\ >\ 0\ ?\ '%h%w%m%r\ '\ :\ ''%}%=%{%\ &showcmdloc\ ==\ 'statusline'\ ?\ '%-10.S\ '\ :\ ''\ %}%{%\ exists('b:keymap_name')\ ?\ '<'..b:keymap_name..'>\ '\ :\ ''\ %}%{%\ &ruler\ ?\ (\ &rulerformat\ ==\ ''\ ?\ '%-14.(%l,%c%V%)\ %P'\ :\ &rulerformat\ )\ :\ ''\ %}]]
+  .. [[ statusline=%<%f\ %{%nvim_eval_statusline('%h%w%m%r',\ {'maxwidth':\ 30}).width\ >\ 0\ ?\ '%h%w%m%r\ '\ :\ ''%}%=%{%\ &showcmdloc\ ==\ 'statusline'\ ?\ '%-10.S\ '\ :\ ''\ %}%{%\ exists('b:keymap_name')\ ?\ '<'..b:keymap_name..'>\ '\ :\ ''\ %}%{%\ &ruler\ ?\ &rulerformat\ :\ ''\ %}]]
+  .. ' rulerformat=%18(%l,%c%V%=%P%)'
 )
 M.nvim_argv = {
   M.nvim_prog,