fix(statusline): default 'statusline' improvements #41597

Problem: there should be exactly one cell of padding between sections,
and exactly one cell of minimum padding between the left and the right.
- Spaces between sections waste space when a section is empty.
  According to a comment in #33036, this was the reason to avoid `%k`
  and implement the keymap section with a vim expression, but other
  sections still have this problem.
- The diagnostics section wastes space because it is not entirely empty
  when there are diagnostics in another buffer.
- The terminal exit code section can touch the right side, e.g. the
  ruler, even though it belongs to the left side.
Solution:
- Use auto-hiding item groups (`%(` without width fields) to get rid of
  unneeded spaces when a section shows no information.
  This simplifies the 'showcmd' and 'keymap' sections in particular.
- As a slight simplification, `term_exitcode` is moved into the flags
  section since it is formatted with square brackets like a flag.
- Count the diagnostics for the current buffer specifically.
- Ensure at least one cell of padding between the left and the right
  side by adding a space next to the separator `%=`.

Problem: some sections are implemented with `%{%`, even though
reevaluation of the expression result is not needed.
This leads to otherwise needless %-escaping in `progress_status`.
Solution: use `%{` instead.

Problem: `%{` and `%{%` (without items) replace spaces with fillchars.
This looks out-of-place inside the terminal exit code section, and in
contrast to all other sections, the 'busy' section is surrounded by
fillchars, which looks inconsistent, and with some terminal-font
combinations, ◐ overlaps the fillchar, e.g. Alacritty & JetBrains Mono.
Solution: use non-breaking spaces U+202F to avoid fillchar substitution.

Problem: sections that appear/disappear frequently can make otherwise
more stable sections jump around a lot.
Solution: sort the sections on the right roughly by volatility:
'showcmd' in first place, 'keymap' next to the ruler.

runtime/lua/vim/ui.lua

@@ -385,7 +385,7 @@ do
         sum = sum + (progress_item.percent or 0)
       end
       local avg = math.floor(sum / count)
-      return string.format('%d%%%%(%d) ', avg, count)
+      return string.format('%d%%(%d)', avg, count)
     end
   end