patch 9.1.1819: Cannot configure the inner foldlevel indicator

Problem:  Cannot configure the inner foldlevel indicator for the
          foldcolumn
Solution: Add "foldinner" suboption value to the 'fillchar' option
          (Maria José Solano).

closes: #18365

Signed-off-by: Maria José Solano <majosolano99@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/fold.txt

@@ -1,4 +1,4 @@
-*fold.txt*      For Vim version 9.1.  Last change: 2025 Jul 15
+*fold.txt*      For Vim version 9.1.  Last change: 2025 Oct 03
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -595,7 +595,8 @@ A closed fold is indicated with a '+'.
 These characters can be changed with the 'fillchars' option.
 
 Where the fold column is too narrow to display all nested folds, digits are
-shown to indicate the nesting level.
+shown to indicate the nesting level.  To override this behavior you can use
+the "foldinner" character of the 'fillchars' option.
 
 The mouse can also be used to open and close folds by clicking in the
 fold column:

runtime/doc/options.txt

@@ -3853,6 +3853,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  foldopen	'-'		mark the beginning of a fold
 	  foldclose	'+'		show a closed fold
 	  foldsep	'|'		open fold middle character
+	  foldinner	none		character to show instead of the
+					numeric foldlevel when it would be
+					repeated in a narrow 'foldcolumn'
 	  diff		'-'		deleted lines of the 'diff' option
 	  eob		'~'		empty lines below the end of a buffer
 	  lastline	'@'		'display' contains lastline/truncate

runtime/doc/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2025 Sep 29
+*version9.txt*  For Vim version 9.1.  Last change: 2025 Oct 03
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -322,6 +322,8 @@ Improvements in 'fillchars':
   "eob" in 'fillchars'.
 - Support for using multibyte items with the "stl", "stlnc", "foldopen",
   "foldclose" and "foldsep" items in the 'fillchars' option.
+- Support for configuring the character used inside a fold level region using
+  "foldinner" in 'fillchars'.
 
 Support for the XChaCha20 encryption method. 'cryptmethod'
 
@@ -41702,7 +41704,9 @@ Options: ~
 - new option values for 'fillchars':
 	"trunc"		- configure truncation indicator, 'pummaxwidth'
 	"truncrl"	- like "trunc" but in 'rl' mode, 'pummaxwidth'
-	"tpl_vert"	- separators for the 'tabpanel'
+	"tpl_vert"	- vertical separators for the 'tabpanel'
+	"foldinner"	- character used inside the 'foldcolumn' for nested
+			  fold levels
 - 'grepformat' is now a |global-local| option.
 - adjust for GTK3 dropping some mouse cursors 'mouseshape'
 - 'nrformats' accepts the new "blank" suboption, to determine a signed or