feat(options): per-buffer 'busy' status #34493

Problem:
Plugins cannot mark a buffer as "busy".

Solution:
- Add a buffer-local 'busy' option.
- Show a busy indicator in the default 'statusline'.

src/nvim/options.lua

@@ -950,6 +950,21 @@ local options = {
       type = 'string',
       varname = 'p_bt',
     },
+    {
+      defaults = 0,
+      desc = [=[
+        Sets a buffer "busy" status. Indicated in the default statusline.
+        When busy status is larger then 0 busy flag is shown in statusline.
+        The semantics of "busy" are arbitrary, typically decided by the plugin that owns the buffer.
+      ]=],
+      full_name = 'busy',
+      redraw = { 'statuslines' },
+      noglob = true,
+      scope = { 'buf' },
+      short_desc = N_('buffer is busy'),
+      type = 'number',
+      varname = 'p_busy',
+    },
     {
       abbreviation = 'cmp',
       defaults = 'internal,keepascii',
@@ -8640,6 +8655,7 @@ local options = {
         '%=',
         "%{% &showcmdloc == 'statusline' ? '%-10.S ' : '' %}",
         "%{% exists('b:keymap_name') ? '<'..b:keymap_name..'> ' : '' %}",
+        "%{% &busy > 0 ? '◐ ' : '' %}",
         "%{% &ruler ? ( &rulerformat == '' ? '%-14.(%l,%c%V%) %P' : &rulerformat ) : '' %}",
       }),
       desc = [=[